Mo Mowlam, the former Northern Ireland secretary, was respected for her frankness. She was blunt in a way that the public found truly refreshing for a top politician. On one occasion she even told Ian Paisley to "fuck off".

It has now emerged, however, that there was one fundamental issue about which "honest Mo", who died in 2005, refused to tell the truth – to the constituents who voted her in and to her boss, Tony Blair. For nine years before her death, she had maintained the fiction that she was suffering from a benign brain tumour when in fact it was malignant. The reality was known only to Mowlam, her husband Jon Norton (who died last year), and her former doctor, Mark Glaser.

Glaser first told Mowlam in 1996, just months before Labour swept to power, that she had a malignant tumour on her brain – and a life expectancy of just three years. After the landslide victory, Mowlam was appointed to the cabinet at the head of the Northern Ireland Office. It was the moment for which she had built her career.

Glaser has revealed the secret to Neil McKay, the author of the Channel 4 drama Mo, starring Julie Walters. He also told McKay that he ordered Mowlam to tell Blair the truth. But she rejected her doctor's instructions, and instead informed both the prime minister and the public that the tumour was, in fact, benign and fully treatable.

Glaser, who is now chief of cancer services at the Imperial College NHS Trust, said he felt a heavy sense of responsibility as Mowlam took up the post of Northern Ireland secretary, in charge of the peace process, denying the existence of a debilitating condition that he expected would kill her within three years.

"A frontal lobe tumour can cause disinhibition, behavioural disturbance and poor judgment," he told McKay. "And there she was taking up a job in what was effectively a war situation. But there was nothing I could do. I was her doctor. I was responsible for her care, even if she wouldn't let me keep records in the proper places or write to her GP."

Glaser describes his plight at the time as a "professional nightmare" and says Mowlam "deceived me". He said: "I told her to tell Blair, but she didn't, she lied. So I went the extra mile for her because she demanded it from me. I didn't seek this. I was trapped. It was a moral issue and a medical issue. She was doing this very important job, one that affected so many people's lives, while she was ill; but she was also my patient and I owed her confidentiality."

The doctor explains how the treatment he administered to Mowlam was, initially, very successful and allowed her to carry on with tortuous and exhausting peace talks until she was moved from the post by Blair, against her will, in 1999.Had she been in the post much longer, Glaser told McKay, he might have had to go to Blair himself to tell him the truth.

While Glaser was troubled by the problems Mowlam posed, he came to like and admire his most famous and difficult patient. He felt by the end of her life that he had become her social worker as well as her doctor. He tells how in advance of particularly important or testing meetings Mowlam would ring and ask to have her medicine adjusted, sometimes to stop her being sick. As time has worn on, Glaser has begun to feel that her illness may, oddly, have been a reason for the success of the peace talks, rather than a cause of instability that threatened them. "She was racing against time," he says.

It is seems clear her tumour may have accentuated and exaggerated her already unusual levels of openness and exuberance, endearing her to the normally flinty, unforthcoming politicians of Northern Ireland who felt more able to open up in discussions as a result.

Even Clare Short, the former international development secretary, a friend of Mowlam's who had a relationship with Jon Norton after her death, was unaware until she saw the drama recently of the real cause of her death. Short had no reason to doubt Norton's explanation that she died from the effects of radiotherapy. She said Mowlam remained very angry to the end at the way Blair removed her from the Northern Ireland job, believing it was the result of a plot hatched by her successor in the post, Peter Mandelson.

Walters, who shaved her head to play the part of Mowlam, says: "The script is very truthful. It is very real."