Twenty years after Asif Ali Zardari shot to political fame when his late wife, Benazir Bhutto, became the first female Muslim prime minister of an Islamic country, the leading candidate to be Pakistan's next president is again battling his past.

The 54-year-old former businessman has spent more than half his 20-year political career in prison in Pakistan fighting corruption charges and most of his recent past in exile fighting off similar allegations in international courts. In the wake of his wife's assassination last December and the decision by former general Pervez Musharraf to step down as president last week there has never been a more pressing moment to present his side of the story.

But the questions facing Mr Zardari are not only over his well-documented fight against various corruption charges but over the scars that those battles left behind.

The scars are documented in a bundle of medical reports dated between June 2005 and September last year on file in Britain's High Court, where Mr Zardari claimed ill health to try to fight off court proceedings brought by Pakistan's former government.

The court case ended last year, along with others brought against Mr Zardari internationally and in Pakistan, after a deal struck between Bhutto and Mr Musharraf that led to her return from exile. A prosecutor in Switzerland yesterday brought a formal close to corruption charges brought against Mr Zardari there.

The sheaf of documents - from specialists ranging from a Dubai cardiologist to a New York psychiatrist - remains, however, and paints a picture of a man with multiple and severe physical and mental health problems.

In March last year, Stephen Reich, a New York state-based psychologist, diagnosed Mr Zardari with dementia, major depressive disorder and post-traumatic stress disorder, problems stemming in part from being tortured while imprisoned in Pakistan. Mr Zardari could remember neither the birthdays of his wife and children, nor more than a handful of facts from two short stories he was read.

"He had difficulty focusing, concentrating and paying attention, is persistently sad, chronically anxious and apprehensive. He stated that he has had suicidal thoughts, but has not made any suicide gestures," Mr Reich wrote.

Another March 2007 diagnosis - by Philip Saltiel, a New York City-based psychiatrist - said emotional and neurological problems suffered by Mr Zardari because of medical treatment and imprisonment had resulted in "emotional instability" and "deficits in memory and concentration". Mr Saltiel wrote: "I do not foresee any improvement in these issues for at least a year."

Mr Reich re-examined Mr Zardari in June and September last year, each time reporting that he had made progress but still had problems that might make it impossible for him to testify in court.

Months after Mr Reich's September diagnosis, Mr Zardari became a key political player in Pakistan after the Pakistan People's party won the most seats in parliamentary elections.

Mr Zardari could not be reached to comment. But his supporters argue that he has overcome his medical problems and is ready to lead Pakistan. "His doctors have declared him medically fit to run for political office and free of any symptoms," said Wajid Shamsul Hasan, Pakistan's high commissioner to London and a long-standing political ally and friend of the Zardari/Bhutto family.

US officials say they can work with Mr Zardari despite the controversies of his past. Indeed, some US diplomats are more concerned about Nawaz Sharif, leader of the junior coalition party PML-N, whose tenure as prime minister in the 1990s they remember as difficult for US-Pakistan relations. Over the past year Washington has made a particular effort to cultivate relations with the PPP.

Mr Zardari's supporters also argue that all of the corruption allegations he has faced have been tainted by politics. "People have to recognise that there isn't even a single case [of corruption] which was conclusively proven against Mr Zardari. The matter has been politicised over the years, people have made wild accusations," said Nayyar Bukhari, a senior leader of the PPP, previously run by Bhutto, of which Mr Zardari is now the co-chairman. PPP leaders also claim Pakistanis have bigger problems than Mr Zardari's past to worry about, such as the growing Taliban-led insurgency and the worsening economy.

Western diplomats in Islamabad say that is true, up to a point. Should Mr Zardari be elected president he will have to demonstrate quickly that he is taking charge of the fight against Islamists, they say. Otherwise his past is likely to resurface as an issue.

"If in the coming weeks or months, there are signs of the Islamic insurgency beginning to be defeated, it is possible that [Mr] Zardari will get credit for that," said one. "The main challenge however is that there are no easy solutions or short-cuts in sight to deal with [militants]. Does this then expose him to criticism on his past? That's the key question." Additional reporting by Daniel Dombey in Washington