Labour defied predictions to score an astonishing victory in the by-election for the Scottish seat of Glenrothes early today, reinforcing Gordon Brown’s recent political recovery.

Its triumph over the Scottish National Party in the constituency that neighbours Mr Brown’s in Fife was the first evidence that Labour’s improvement in the polls appears to be real.

The scale of the win, however, took everyone, including Labour strategists, by surprise. It marks an abrupt end to Alex Salmond’s honeymoon as Scotland’s First Minister and a thumping personal endorsement for Mr Brown.

Holding the seat that Labour won with a 10,664 majority in 2005 meant that Mr Brown’s decision to break with tradition and campaign in the contest — along with his wife, Sarah, who made several visits — paid off.

Lindsay Roy, 59, secured the seat for Labour with 19,946 votes — a majority of 6,737. The SNP came second with 13,209. Maurice Golden, the Conservative candidate, was third with 1,381, and Harry Wills, of the Liberal Democrats, was fourth with 947.

Labour officials had deliberately played down expectations in recent days, while the SNP had made confident predictions of victory. The Labour win was all the more remarkable because the corresponding seat was taken by the SNP, with a comfortable majority, at the Scottish Parliament election last year.

Mr Roy, headmaster of Mr Brown’s old school, held off a fierce challenge from the Peter Grant, of the SNP, to increase Labour’s working majority in the Commons to 63. Labour had mounted a ferocious effort yesterday to get out the party vote and may have been helped, however slightly, by news from London that interest rates had been cut by 1.5 percentage points.

In his victory speech, Mr Roy thanked Mr Brown times and said:

“In these difficult economic times, I pledge my support to the leader of our country. He’s somebody who has worked in support of all of us not just in Fife, but in Scotland and in the UK during these volatile economic times.”

The by-election in the Fife constituency was following the death of sitting Labour MP, John MacDougall.

Mr Brown’s improved performance in recent weeks — he has won plaudits for his handling of the financial crisis — had meant that a defeat in Glenrothes would not have renewed immediate questions over his leadership. Less than two months ago it looked as if defeat might have brought about an attempt from within the Cabinet to remove Mr Brown from office.

That prospect has evaporated after Mr Brown used a Cabinet reshuffle to see off the threat to him from Blairite ministers and to strengthen his Commons base. The return of Peter Mandelson, though fraught with risk, was key to removing the possibility of an internal challenge, as was the offer to John Hutton of the position of Defence Secretary that he sought.

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