The Prime Minister's wife spent the weekend back on the campaign trail in Glenrothes as the fight for the crucial by-election enters its final days.

Sarah Brown returned to knock on doors once more, both yesterday and on Saturday, speaking to voters in the area in a bid to boost Labour's chances.

A Labour spokesman confirmed that Mrs Brown had made several low-key visits during the political battle for the Fife constituency - which is next door to her husband's Kirkcaldy and Cowdenbeath seat.

The spokesman said: "She's just quietly been talking to people on the doors and getting on with it."

The move follows a high-profile visit to the constituency by Mrs Brown earlier in the campaign when she said she was "very pleased" to be supporting Labour's candidate Lindsay Roy.

Her visits, along with those of her husband, have been taken as a sign of the seriousness with which Labour is treating the by-election.

The contest in Glenrothes is taking place after the death of Labour MP John MacDougall, who had a majority of 10,664 at the last election. That is smaller than the majority the SNP overturned to win the Glasgow East seat in the summer.

However, a survey by a Scottish Sunday newspaper yesterday put Labour ahead in Glenrothes, with 26.5% of those questioned backing Labour compared to 23% for the SNP.

Almost one-quarter of those quizzed said they were undecided, however, suggesting that the looming contest is still wide open.

First Minister Alex Salmond maintained that he was confident of a victory for Peter Grant, the SNP candidate, on Thursday. He was quoted yesterday as saying: "My prediction is we'll win it."

He added: "I have never subscribed to the view of three months ago that Glenrothes is an easy win for the SNP. It's not.

"But the reason for confidence is that we won the Central Fife seat at the Scottish elections. So we are not dealing with an area where there hasn't been a substantial amount of SNP activity or information and organisation.

"I don't anticipate we shall win by a landslide. But you ask me the question Do I think we will win?' The answer is Yes I do'."

This weekend, bookmaker Ladbrokes was offering odds of 5/6 on both the SNP and Labour to win the seat.

William Hill has also shortened the odds on Labour, making the party 5/6 joint favourite along with the SNP.

William Hill spokesman Graham Sharpe said: "All the early money was for the SNP, but now punters have begun backing Labour as though defeat is out of the question."