KEZIA Dugdale has reached out to undecided voters with a "simple, honest plan" to raise taxes in order to invest in schools and prevent further cuts to public services.

The Scottish Labour leader sought to woo the 25 per cent of Scots who have yet to make up their minds how to vote on May 5 as she launched her party's manifesto in Edinburgh.

It came as a new poll suggested Labour was on course to come third in a national poll for the first time in more than 100 years.

The Ipsos MORI survey for STV news gave Labour a slight lead over the Tories in the constituency vote but put the party two points behind in the all-important regional ballot.

According to the widely-used ScotlandVotes online seat predictor, that would give the Conservatives 23 seats to Labour's 20.

The SNP remained on course to form a majority government, with 71 out of 129 seats, while the Greens were set to overtake the Liberal Democrats.

If Labour is overtaken by the Tories, it will be the first time the party has finished third in a national election since 1910 and, despite her determination to carry on in the job, could raise questions about Ms Dugdale's short leadership.

Unveiling Labour's manifesto in the same Edinburgh community centre where she launched her bid for the party leadership last year, Ms Dugdale made passionate plea for undecided voters to back plans which would see all the least well off Scots taxed more, but which she said would prevent fresh spending cuts.

After a number of confused attempts to position Labour on the question of a possible second independence referendum, she also offered her most uncompromising pledge yet to defend the Union.

Making an "unequivocal" promise to oppose a second vote, she told supporters she part in a "hard shift" campaigning for No in 2014 and felt "just as strongly" about Scotland remaining in the UK.

"I don't want ever to have to participate in another referendum.

"I believe that question has been answered," she said to applause from her audience of around 150 supporters

In a direct challenge to the Scottish Conservatives - who have made defending the Union their main election issue - she added: "I take real umbrage at any suggestion you can trust the Tories with the Union.

"The Tories don't defend the Union, they are the risk to the Union."

Labour's launch, in a dingy, windowless community hall, was in stark contrast to the SNP's manifesto rally at the Edinburgh International Conference Centre attended by 1400 cheering activists last week.

At the heart of the 65-page document was Labour's plan to raise the basic and higher rates of income tax by 1p and the additional rate, paid by those earning over £150,000, by 5p, in order to prevent further cuts to public services.

Labour say the move will raise an additional £1.2billion per year by 2021, compared with the £300million extra raised under the SNP's more cautious plan to limit George Osborne's tax cuts for the better off south of the Border.

Ms Dugdale said she was "appalled" the Nationalists had failed to use Holyrood's new tax powers - the parliament takes control of income tax from April next year - to counter the UK Government's austerity programme.

In a heartfelt appeal to undecided voters, she summed up her own proposals in just 22 words.

She said: "As many as one in four voters at this election are undecided. "I want to speak directly to them as they make up their mind.

"If you are still unsure how to vote, here is our plan.

"We'll tax the richest one percent, so we can invest in schools and stop the cuts to our NHS and public services."

Among the specific policy pledges, the manifesto confirmed a widely-trailed pledge to fund breakfast clubs in all primary schools as part of a "childcare revolution".

Other key pledges include guaranteeing an appointment at a GP surgery within 48 hours, replacing the "unfair" council tax with a system based on property values; keeping university tuition free and reversing cuts to student grants.

Ms Dugdale said Labour was also the only party promising to maintain spending on the arts, in real terms, over the next five years.

Responding to the poll, Scottish Conservative leader Ruth Davidson said: "We now intend to work harder than ever over the next week to show people we can be the strong opposition to the SNP our country needs."

Derek Mackay, the SNP's business convener, said: "This is another very encouraging poll for the SNP which shows that our positive message has strong support on the doorsteps – but we take absolutely nothing for granted and are working hard for every vote."