The 58-year-old headmaster of Gordon Brown's school was chosen last night to be Labour's candidate in the forthcoming Glenrothes byelection, a contest which could prove crucial to the Prime Minister's future.

Lindsay Roy, the head of Kirkcaldy High School who has known Mr Brown for 15 years, was selected ahead of two other nominees to defend Labour's 10,664 majority against a strong challenge from the SNP.

Although no date has been set for when the by-election will be held, it is expected around the end of next month or beginning of November. It has been triggered by the recent death of the sitting Labour MP John MacDougall after a long illness.

The contest in a seat that borders Mr Brown's own seat in Kirkcaldy and Cowdenbeath and is only 20 minutes' travel time from his home in North Queensferry comes after Labour's catastrophic loss to the SNP of the Glasgow East by-election in July.

In Glasgow East, the SNP achieved a 22 per cent swing to overturn Labour's 13,507 majority - far more than the 14.5 per cent needed in Glenrothes.

The by-election will come shortly after the Government's “autumn fightback”, which will include a package of financial measures aimed at hard-pressed families and easing the mortgage crisis as well as Mr Brown's Labour conference speech, seen as fundamental to his chances of turning round his own and his party's prospects.

If Labour loses the seat in the Prime Minister's political backyard, it will leave Mr Brown as the last Labour MP in its once rock-solid Fife powerbase, and put further pressure on his leadership as well as raise again the prospect of a challenge.

The Times, which reported last week that Mr Roy was the frontrunner to be selected, has also been told that Mr Brown's Cabinet colleagues are urging him to campaign personally in Glenrothes, thus breaking the convention that prime ministers do not become directly involved in by-elections.

There is also evidence that Mr Brown is taking a personal interest in how the Labour campaign is run. One Labour-supporting Scottish tabloid claimed last week that he was even involved in finding a suitable candidate.

Mr Roy is seen as a formidable and articulate candidate who was appointed CBE in 2004 for his services to Scottish education. He has been a member of the Labour Party in Fife for 20 years, and was drafted in to his present post at Kirkcaldy High only in May this year after the school received a damning report from inspectors, who said that it was suffering from poor staff morale and pupil indiscipline.

He was headhunted by the local authority, which is now led by an SNP-led coalition, to go to Kirkcaldy High, making him, Labour believe, a much more difficult target for the Nationalists to attack. Mr Roy was previously head of Inverkeithing High, also in Fife, which received consistently glowing reports from the inspectorate.

One source in Labour circles in Fife said: “Lindsay has a great track record in education in this area and is really well-respected and well-known locally. He'll be an excellent candidate. His commitment to the seat and the area is really strong.”

The SNP selected ten days ago Peter Grant, the leader of the Nationalist-led Fife Council, as its candidate while the Liberal Democrats will field Harry Wills. The Conservative candidate will be Maurice Golden.

The campaign themes are expected to repeat Glasgow East, with Labour concentrating on local issues while the SNP turns the spotlight on the economic gloom. The bookmakers are similarly backing the SNP, making the Nationalists 1-4 on to take the seat.

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