We are in the headquarters of the International Leukaemia Foundation, the charity he founded in 1988, one year after his own celebrated recovery from the disease.  The building is a temple to modern Spanish style, with modish artworks on the ochre walls and fabulous, sandblasted glass doors.  There is a profusion of black leather chairs - with cushions as slim and elegant as envelopes - and a stunning boardroom table, designed by Pete Sans.

Clip-clopping across the waxed parquet floors in groovy ankle boots, Carreras is something of a dinky design statement himself, dapper as any Catalan businessman in his navy wool jacket and grey flannels.  He loves shopping for clothes, but is not confident about his choice of garments. "I try to make a nice match," he says, pointing to his blue shirt and icy green tie, "but I am not very good at this."

He is surprisingly tiny, with slim shoulders and none of the barrel-chested vigour normally associated with top tenor singers. "  My chest is not that small, come on," he exclaims, flapping open his blazer and sounding rather like a peeved beauty contestant.

"OK, I am a small man," he concedes, "but you sing with the muscles, not with the fat.   You don't need to be big to sing.  Size has got nothing to do with the voice."

He is 50-years-old and tries to keep as fit as possible, with regular tennis, walking and swimming.   Nevertheless, he does have that pale, papery look of someone who was once diminished by a serious illness. Although still remarkably handsome - particularly in profile - his hair recedes oddly and his skin, bereft of any rosy bloom and untouched by the Mediterranean sun, looks fragile and translucent.

In manner, he is courtly and achingly polite, but he cannot stop his emotions dancing across his face as clearly as if they had been written on his forehead in ballpoint pen - cheery, bemused, amused, heated.  In repose, however, his features seem to fall in on each other, and he looks preoccupied and sad.

Every two months, he has routine tests to check for any recurrence of leukaemia, and he is pleased with his progress.  "I am so happy because I am out of any treatment, any therapy, and I live a completely normal life without limitations," he says, his face lighting up with a dazzling smile.  "I have a 100 per cent green light from my doctors, which is fantastic."

In his corner office in the Foundation, his briefcase is tucked neatly by his chair and his desk is oddly empty of any paperwork whatsoever.  He seems to spend a great deal of time on the telephone, halting the flow of business calls only when informed that his 19-year -old daughter, Julia, is holding on line two.  "Ahh. The most important person," he beams, as she is patched straight through.

After the call is finished, he tells me proudly that she begins a university course in biology this September, and that his son Alberto, 24, is studying to be a lawyer.  He is a devoted father and has taken pains to retain some semblance of family togetherness since divorcing Mercedes, the mother of his children, in 1992 after 22 years of marriage.  Today, they enjoy what he describes as a "cordial" relationship.

"It is very civilised; we respect each other a lot.  We meet often and we spend every Christmas together with the children.  She is a wonderful person," he explains, "but the life between us as a couple did not work."

He has been associated with many beautiful women, most famously with the Italian soprano Katia Ricciarelli, with whom he sang often in the Seventies.  "They were," says one opera critic, "the Alagna and Gheorghiu of the day."   After this, he had a long relationship with an Austrian air hostess, Jutta Jaegar, and, more recently, with a Polish model, Patrycja Woy-Wojciechowska.

He is notoriously reluctant to talk about his relationships, but he readily admits that he is a "compulsive" man.

"I think more with the heart than the head, but I love myself for these mistakes.  It is me. You see, sometimes I am weak in certain situations.  I am spontaneous.  I... Ahhhh...How can I express this?"  He struggles for the English words - or perhaps he is playing for time, realising what dangerous territory he is heading into.  He smiles, then changes tack slightly.

"I do not like to face confrontations, personally or professionally.   It is a little kind of weakness in my character.  I get too emotional." 

What is he saying here?  "I am saying that I hate people to get angry with me."

Hmm.  Men who describe themselves as spontaneous are sometimes merely the self-indulgent types who invariably end up in emotional hot water.  So is he the type of person whom people - women in particular - get angry with?

"It depends on the woman.  Ha! I think it is lovely to be spontaneous.  If sometimes you get a bit emotional, it is healthy. It is necessary for your mental and physical well being.  I am from the south, from Barcelona.  I know a lot about this.  I am a little bit over the top.  I am too impulsive.  I am not any more open to be rational.  Sometimes, I am throwing chairs around."

Really? That sounds alarming.

"Only for one-and-a-half minutes," he says.  "And usually to do with Barcelona football team."

The phone shrieks; another business call, a timely distraction.  Each year, Carreras raises $1 million for the Foundation by donating a portion of his fees from a dozen or so concerts.  This helps fund the medical research that is the focus of the Foundation's work.  "The type of treatment I had performed on me 10 years ago is pre-history, compared to what is going on nowadays," he says.

At the height of his illness, he underwent chemotherapy, radiotherapy, a bone marrow transplant and months alone in a sterile hospital room.  During the transplant operation, he refused a general anaesthetic because of the damage the tubes could inflict on his vocal cords.

He was groggy, but awake when doctors punctured his pelvic bone hundreds of times to remove a litre of marrow.

The operation took place in Seattle, in the United States, partly because of the excellence of the facilities available there, but also because of what Carreras calls "social pressure" at home.  When he was first hospitalised in Spain, television bulletins about his progress were broadcast three times daily, and women queued outside, offering to donate their own bone marrow.

Today, he would react to this by enthusiastically encouraging them to join his worldwide list of possible donors.  "Now, I have a real calling in my life," he says. "The Foundation is the most positive thing that came out of a very severely difficult period.  Now, I know what I am going to do the day I stop singing.  I am going to dedicate my efforts and my energy to this."

When does he think he will stop singing?

"Oooooooo," he says, sucking in his breath and looking troubled."    Difficult.  A very difficult matter.  I think that the audience will tell me.  At the moment, my instrument - my voice - is still OK and I still feel that there is something inside myself that I can share with everyone."

But not everyone agrees.  Collective critical opinion seems to suggest that Carreras's rich and liquid voice - he was born with an extraordinary natural talent - was past its best before he became so terrifyingly ill.  Although he was called the most gifted lyric tenor since Carlo Bergonzi, his early indulgence in heavy operatic roles - Verdi's Radames and Bizet's Don José, for example - are believed to have exacted a toll.

He was encouraged in some of these endeavours by the powerful and dictatorial Herbert von Karajan, and many believe that his voice never fully recovered.

By the mid Eighties, many reviews of Carreras's performances and recordings were including words such as "overcooked", "tight" and "bleating".  Does he feel now that he took the wrong career path?

"I know what you mean.  Some people said that this was wrong and it was bad for me, but much of the works I did with Karajan were the greatest successes in my career.   So, therefore, who is right? Who knows? I do not regret what I did.  I made mistakes, I am still making mistakes.  Maybe I would do it differently today.   I just don't know."

Does he feel that the stress of this difficult and exacting work load contributed to the onset of his cancer?

"I honestly do not think so.  I know that there are many different opinions about how psychosomatic certain types of diseases are, or how the type of life you lead can contribute.  In my case, I think it was purely physical, probably genetic," he says, pointing out that both his mother and his uncle died of cancerous diseases.

He made his astonishing comeback in Barcelona in 1989, singing to 150,000 people on an emotionally charged evening.   "It was one of the most important, greatest moments in my life.  And if it was too soon after my illness, can I please say: Who cares? It was better than any medicine, it was good for my spirit."

In the beginning, he had no ambition to be a famous opera singer; he just wanted, he says, to be "good, a part of the game".

He could not possibly have imagined that, many years later, he would be one of the best -known singers on the planet.  After his illness, he crested on a wave of popular support and affection, enjoying a career renaissance of sorts when he teamed up with Luciano Pavarotti and Placido Domingo to form the Three Tenors.  The idea was his own, launched initially as a charity gala - part of the 1990 World Cup festivities.

"But to do it for every World Cup would be wrong.  You must not look at these things in a very commercial way.  It has to be," he says, reaching for his favourite word once more, "spontaneous."

Unlike Pavarotti and Domingo, he keeps to a light schedule and is no longer a regular in the great opera venues.  He has just appeared in Wolf-Ferrari's Sly - an adaptation of The Taming of the Shrew - in Zürich, and hopes to sing again at the Met in New York and London's Covent Garden.

"It is not over for me yet," he insists.   "There are many thins still to do.  I consider myself a very lucky person, for obvious reasons.    I am full of enthusiasm, my spirit is good.  As a man, I feel the same as before - a little bit more mature, but making the same mistakes as always.

"Is that such a bad thing?   I don't think so."

José Carreras gives his only UK performance this season at the Hampton Court Palace Festival tomorrow at 7.45pm.  Tickets from Ticketmaster: 0171-344 4444.

Copyright © 1998 Electronic Telegraph