In answer to a direct question from interviewer Andrew Marr, he denied that he was dependent on prescription painkillers.

Rumours about the Prime Minister's possible use of painkillers have circulated in Westminster following speculation on an internet blog, but Mr Brown's interview on BBC1's Andrew Marr Show this morning was the first time that he has been confronted with them in public.

Some unconfirmed media reports have recently suggested that Mr Brown might use concerns about his health as a reason for stepping down as Prime Minister ahead of the election to allow Labour to chose a new leader to take it into the campaign.

Marr today told Mr Brown he wanted to ask about "something everybody has been talking about in the Westminster village... A lot of people in this country use prescription painkillers and pills to help them get through.

"Are you one of them?"

The Prime Minister responded: "No. I think this is the sort of questioning which is all too often entering the lexicon of British politics."

Mr Brown went on to discuss the well-documented problems he suffered as a result of a boyhood rugby injury which cost him the sight in one eye, revealing that he recently underwent an annual check-up which showed no deterioration in the vision of his good eye.

When Marr tried to steer the interview back onto the issue of painkiller use, the PM cut him off, saying: "I've already answered that question."

Discussing his eye injury, Mr Brown said: "I have had very serious problems with my eye. I lost my eyesight playing rugby. I had three major operations and they couldn't save my sight.

"I then had exactly the same thing happen in my second eye. I had the same retinal detachment. I had the same fear that therefore I might lose my sight in that eye.

"I had to have a very big operation to deal with that and every year, of course, I have to check - as I did only a few days ago - that my eyesight is good.

"There has been absolutely no deterioration in my eyesight. I think people should be absolutely clear that although I had problems with my eyes and it has been very difficult over the years, I think people understand that you can do a job and you can work hard.

"I think it would be a terrible indictment of a political system if people thought because you had this medical condition, you couldn't do a job."

He added: "When people ask these questions about these things, particularly my eyesight, I feel I have done everything to show people that I can do the job even with the handicap that I have had as a result of a rugby injury."

Lord Mandelson told a rally of the Progress thinktank in Brighton yesterday that Labour should demand from the media "equal scrutiny of what other politicians are saying and the same degree of personal intrusiveness that we saw the Prime Minister receive from Andrew Marr this morning on his programme, based on what I have not the foggiest idea and I wonder if Andrew did either".

Anger in the party about Mr Brown being subjected to quizzing about his use of prescription pills was reflected by former minister Caroline Flint, who today branded the question "despicable".

Ms Flint - who quit the Government in June complaining of being treated as "female window dressing" - told GMTV: "Gordon does look tired sometimes. He is not necessarily the most photogenic person.

"I understand this rumour about Gordon's health was because of something put online and I think that sort of gossip and innuendo is pretty despicable.

"I don't think there is any truth in this gossip and I think it is really quite despicable. I think it is just very unhelpful in politics to have that sort of smears and innuendoes put about.

"He is doing a tough job and sometimes looks knackered, but he has got a tough job to do."

Left-wing Labour MP Alan Simpson described Mr Brown as a "dead man walking" and suggested he had until Christmas to convince the party to allow him to stay in post for the election.

Mr Simpson said the left of the party - which has so far largely stood aside from plotting over the leadership - would not "stand aside" if it seemed Labour was doomed under Brown.

The Nottingham South MP told the Evening Standard: "Gordon's conference speech is really dead men walking. The conference is a pageant of loyalty, behind the scenes there is a mayhem of complaints.

"When you have senior Cabinet ministers all acknowledging that the morale of the party is rock bottom, what they are recognising is that grumpy old men have difficulty raising morale.

"If the haemorrhaging has not been staunched, then I think the Left would not just stand around and watch the party bleed to death from self-inflicted wounds.

Mr Simpson said Downing Street had a "last days of Hitler bunker mentality", adding "Gordon has until Christmas".

But fellow left-winger Diane Abbott, Labour MP for Hackney North and Stoke Newington, told the Standard: "I don't know of anybody on the Left saying that we should move against Gordon in the New Year. My own view is we have to soldier on."