THE SNP has paraded a former Labour candidate for Holyrood as one of its newest members, as the nationalists stepped up their drive to cement a commanding lead in the polls by winning the support of undecided voters.

Linda Millar, a trade union activist who was a list candidate for the West Scotland region in 2011 and a former member of Labour's Scottish Executive, said she had joined the SNP as she believed it was the only party offering a "positive vision for Scotland".

The nationalists revealed Ms Millar had joined their ranks as Nicola Sturgeon, sporting an outfit in a shade of what she described as "SNP red", spent the morning in Gordon Brown's back yard of Kirkcaldy, which is on paper one of Labour's safest seats.

It came days after Ms Sturgeon launched a "SNP guarantee" to traditional Labour voters, in a bid to reassure them about switching to her party and emphasising its left wing credentials.

Ms Millar, 58, was placed fifth on the Labour party list in the West Scotland region in 2011, with the top three candidates eventually winning seats at Holyrood. She quit the party last year, before joining the SNP last week with her husband, Ramsay Millar, who had been a Labour activist for four decades.

She said: "For some time I have been concerned with the direction Labour in Scotland has been going, and do not feel that this is the same Labour Party I joined 25 years ago. They are no longer representing the aims of social justice for working people like myself and my family.

"Last week I made the decision to join the SNP with my husband. We have been hugely impressed with Nicola Sturgeon, and believe that a strong group of SNP MPs will deliver real change.

"We both believe that if we want a better future for our children and grandchildren, the SNP are the only party offering a positive vision for Scotland. We are encouraging all our family and friends to vote for the SNP on May 7th, and for the Scottish Parliament Elections next year."

In 2010, Labour won a majority of more than 23,000 in the Kirkcaldy and Cowdenbeath constituency, the fourth largest margin in Britain, when Gordon Brown was the candidate. However, Mr Brown has retired and a Lord Ashcroft poll published last month detected a huge swing to the nationalists, finding the SNP ahead by six points.

The First Minister said: "More than 400,000 people are journeying from voting Labour in 2010 to voting SNP next month, and it is very encouraging that this includes a former parliamentary candidate and such stalwarts as Linda and Ramsay."

While the SNP hold a strong lead in the polls, Labour believes a large number of undecided voters could swing behind Jim Murphy and help stem the nationalist tide.

A Scottish Labour spokesman said hundreds of Labour and trade union activists were campaigning daily for an Ed Miliband led Government which would be one of the most progressive administrations ever in the UK. "The reality is that every vote for the SNP makes it easier for the Tories to hang on to power," he added.

While visiting offices for Barnardo's in Fife, Ms Sturgeon also turned her sights on the Conservatives and Liberal Democrats who she said could not be trusted to keep their promises over more powers for Holyrood. The UK Tory manifesto, published on Tuesday, was described by both the SNP and Labour as a betrayal of the Smith Commission.

She said: "George Osborne's long-term economic plan is penalising the most vulnerable in our society. It has held back economic growth and it's undermining our public services, and only the SNP offers a clear alternative to austerity in this election. What the Tory manifesto proves is that the Tories can't be trusted to deliver more powers to the Scottish Parliament."

She also accused the Liberal Democrats, which polls have predicted are on course to see their number of Scottish seats fall from 11 to one, of representing "everything that has been wrong with the last Westminster government".

She added: "For most people across the country the Lib Dems represent dishonesty - they broke their promise on tuition fees. They represent cuts - they helped the Tories impose painful austerity cuts.

"The Liberal Democrats have been part and parcel of everything the Tory government has done and they will pay the price for that at the next election."