John Prescott will be drafted into the by-election contest in Glenrothes tomorrow in a last-ditch bid by the Labour Party to convince undecided voters in the constituency to back Gordon Brown.

The presence of the former deputy prime minister on the final day of campaigning before Thursday's vote illustrates how close the contest is, although last night bookmakers said that the SNP had regained the title of slight favourites.

Mr Prescott said that he would spend the day in the Fife seat attacking the local SNP-led council, which is headed by Peter Grant, the SNP's by-election candidate, over rises in home care charges.

He will also portray Lindsay Roy, the Labour candidate and headmaster of the Prime Minister's former school, Kirkcaldy High, as a source of inspiration for young people in the area.

Mr Prescott claimed that the outcome of the by-election was still important to him even though he is now out of frontline politics. The SNP remain sure that they can overturn Labour's 10,664 majority at the 2005 general election.

Alex Salmond, the First Minister and SNP leader, reiterated his conviction that his party would win. He told the Daily Politics programme on BBC2 that steeply increased energy bills, which are now beginning to land on people's doormats, were a major factor in the contest and one on which the Nationalists have relentlessly campaigned.

Mr Salmond said an SNP victory would advance the case for independence but would not change the party's timetable for a referendum.

“The issue of the constitution isn't going to be settled in a single election contest, it has to be settled in a referendum,” Mr Salmond said.

“Because - quite rightly - many people will be voting SNP this Thursday because they are shocked and disgusted by paying 30 per cent, 40 per cent - in one case I even saw 50 per cent - more on their gas bill at a time when energy prices are falling, and they are looking for something to be done about it.”

The SNP also claimed that Labour's recent momentum, which has, by common consent, seen them close the gap on the Nationalists, had peaked and begun to run out of steam.

The party said that the decision to send more than 1,000 SNP activists into the constituency over the weekend would prove decisive. “We are out-organising a downbeat and negative Labour party,” an SNP spokesman said.

The Nationalists have resurrected the slogan “Yes, We Can”, which they deployed in the 1997 general election campaign. “It's positive, upbeat and optimistic”, a spokesman said.

So important has the result of the by-election become for the Prime Minister, who holds the neighbouring seat of Kirkcaldy and Cowdenbeath, that he is keeping in close touch with developments during his visit to the Gulf states.

Yesterday, it emerged that he had sent a personal letter to 15,000 voters in the constituency urging them to back Mr Roy.

The letter warned against independence and vowed to steer the country through the current troubled economic times.

He also spoke of his pride in being from Fife and added: “I will do all it takes to save jobs, keep food and fuel prices low and bring stability back to our financial system.”

Meanwhile , it emerged that a completed ballot paper was addressed by mistake to one of the candidates in the by-election.

The postal ballot paper should have gone to election officials at Fife Council but instead it went to Kris Seunarine, the UK Independence Party candidate.

“I was shocked to receive someone else's completed ballot paper in my mail,” said Mr Seunarine, who passed it on to election officials.

Iain Grant, the deputy returning officer, said: “It appears that the elector did not follow the instructions regarding the return of their postal vote.”

He said the voter had put the ballot paper in the envelope in such a way that Mr Seunarine's address, rather than that of the election office, was visible in the address window.

“I can now confirm the ballot paper has been handed to the elections office and will be dealt with along with other postal votes,” he said.

The Liberal Democrats, in third place in the seat in 2005, will today wheel out two of their big party guns - Sir Menzies Campbell, the former party leader and himself a Fife MP, and Vince Cable, the party's economics spokesman in the Commons.

Bookmakers William Hill said last night that the SNP are now 4/6 favourites to win the by-election. Labour have lengthened from 5/6 joint favourites to 11/10. Hills offer the Conservatives and Lib Dems each at 100/1.

Fight for power

— The population of Glenrothes is 40,000. Labour won the seat at the 2005 election with 19,395 votes, 10,664 ahead of the SNP

— The SNP won Central Fife, which covers the same area, in last year’s Scottish elections by 1,166 votes

— The Nationalists need a swing of 14.2 per cent to win on Thursday. The party won the Glasgow East by-election in July with a swing of 22.5 per cent from Labour

— The seat combines Glenrothes new town with a section of the Fife coalfield to form a mostly working-class constituency that stretches inland from the north coast of the Firth of Forth

— After Glenrothes, its main urban centres are the coastal industrial towns of Buckhaven and Methil

— The candidates are: Morag Balfour (Scottish Socialist Party); Maurice Golden (Conservative), below; Peter Grant (SNP); Louise McLeary (Solidarity); Jim Parker (Scottish Senior Citizens Unity Party); Lindsay Roy (Labour); Kris Seunarine (UK Independence Party); Harry Wills (Liberal Democrat)

Times Recommends

• Secret files show Thatcher's role in coup plot
• Veterans to boycott Shell after ban on poppy sales
• 'I survived so please let Baby RB live too'