At the weekend his name was linked in the media with a married woman he was said to have met at a clinic in the Borders specialising in alcohol problems.
In a letter to his local Conservative association chairman, Ian Muir, Mr Stewart said that after discussing his position with his wife, Susie, he had decided to resign as prospective candidate for Eastwood, near Glasgow.
"I do so with great regret so near to a general election, but I am sure that the association will be able to select an excellent candidate and achieve a resounding majority," Mr Stewart said.
After the media reports, in a Scottish Sunday newspaper last weekend, he told members of his constituency that he wanted to resign and they met last night to finalise the matter.
The 54-year-old MP had a majority of 11,688 at the 1992 election, the safest Tory seat north of the border. Until 1995, he was a junior Scottish Office minister, a role that ended after he was fined for a pickaxe confrontation with anti-motorway protesters on the site of the new M77. He resigned his post, writing to John Major to say he did not want "in any way to be an embarrassment to a government I have been proud to serve".
Mr Stewart's views were seen as a litany of Thatcherite principles: a staunch Unionist, an anti-devolutionist, in favour of hanging, and an undiluted market-forces man.
He is also a libertarian non-smoker who has loudly defended the freedom of smokers to poison themselves if they want to.
But after Mrs Thatcher made him a Scottish Office minister, MPs found his performance at the Despatch Box less confident than his extravagant mutton-chop whiskers might have suggested. Mrs Thatcher dropped him from the Government's ranks during a major reshuffle in 1986.
Mr Muir said last night: "There should be no glee on the part of our political opponents. This is a human tragedy of immense proportions which will touch the hearts of all but the most hardened."
The association will meet later this week to choose a successor candidate.Reuse content