A third of Scots would be less likely to vote SNP if the party promises another independence referendum in its Holyrood election manifesto, according to a new poll.

The Survation poll for the Daily Mail found that such a pledge would make 31% more likely to vote for the party while 36% said it would make no difference.

It comes after First Minister Nicola Sturgeon yesterday said the SNP will set out the timescale for a possible second referendum on independence in its manifesto for next year's Holyrood election.

Scotland's First Minister confirmed the party's manifesto will include more details on the circumstances under which another vote might be ''appropriate''.

The study also found a slim majority of Scots still support the union.

More than half (51%) of those questioned said Scotland should not be an independent country while 49% backed independence, when undecided voters were removed.

The poll of 1,010 Scots, carried out from September 7 to 10, found more than half (51%) supported staying in the European Union.

It also questioned respondents on Holyrood constituency voting intentions and found that 53% planned to vote SNP followed by 22% for Labour.

Meanwhile, 14% said they would vote Conservative, 6% Liberal Democrat and 5% other.

A separate YouGov poll for The Times found that, on Holyrood headline voting intention, 51% would vote SNP, 22% Labour, 18% Conservative, 5% other and 4% Lib Dem. This excluded those who said they did not know or would not vote.

Support for Ms Sturgeon appeared strong, with 67% saying she is doing well as First Minister and 28% saying she is doing badly.

Almost two-thirds (63%) think David Cameron is doing badly as Prime Minister while 30% think he is doing well.

Meanwhile, more than a third (35%) think Kezia Dugdale is doing badly as leader of the Scottish Labour Party while a fifth (20%) think she is doing well.

Almost two in five (38%) think Ruth Davidson is doing well as leader of the Scottish Conservatives while the same proportion think she is doing badly.

Most of those questioned in the survey of 1,110 Scottish adults do not think the Scottish opposition parties have been effective at holding the Scottish Government to account.

The survey was carried out between September 7 and 10.

Ms Davidson said: "Labour is now a divided force across the UK, and a declining force across Scotland.

"Voters gave their verdict in May but, instead of listening to their concerns, Labour's response has been to retreat to its own comfort zone with policies which don't represent the values of Scotland's majority.

"Meanwhile, the SNP has gone back on its promise to respect last year's referendum, and is instead now preparing for a re-run.

"Scots don't want to go back to another divisive referendum, they want to crack on with their life.

"Under my leadership, the Scottish Conservatives will always make the positive, principled case for our United Kingdom and will fight in the centre-ground for better public services across Scotland.

"Scotland's two million No voters need a strong voice at Holyrood and I'm determined to be that voice.

"While Labour and the SNP veer off to the extremes, we will stay rock solid in the centre of Scottish politics - building on the clear decision we made in last year's referendum to stay part of the UK, keeping our defences strong so we can be a force for good in the world, and standing up for family finances against demands for tax hikes."

A Scottish Labour spokesman said: "Scottish Labour is changing and politics is changing. Under the new leadership of Kezia Dugdale we are asking people to take another look at the Scottish Labour Party.

"We have positive ideas to build a country where the potential of every child in every classroom is unlocked and we close the gap between the rich and the rest.

"It will take time for people to hear our message but we will leave no one in any doubt that Scottish Labour is for everyone who wants to see a fairer Scotland."

Green MSP Alison Johnstone said: "Patrick Harvie and I have worked hard to challenge the Scottish Government on key issues but have also sought to be constructive to take Scotland forward.

"We pushed for a ban on fracking, which led to the SNP's moratorium; we called for energy efficient housing to become a national infrastructure priority, which the finance minister then agreed to do; and we proposed the right to buy your club for football fans, which the Government then put into legislation.

"With a bigger Green team at Holyrood we can build on these successes. We must create more and better jobs in our economy, and support Scotland's small businesses.

"We must prioritise our neglected colleges and relieve pressures on our NHS. And we must challenge the failure to meet climate change targets and the uncertainty facing communities being targeted for fracking and coal gasification."

SNP business convener Derek Mackay MSP said: "These polls are very encouraging news for the the SNP. After a full eight years in administration, it shows that the party's record of delivering in government is paying dividends with the public showing their trust in Nicola Sturgeon and her team.

"What these polls also make clear is that this is a real low point for Labour, with the Tories now considered more effective than Kezia Dugdale's Holyrood group. After Ms Dugdale openly attacked Jeremy Corbyn and then cosied up to him when it became clear he would win the UK Labour leadership contest, it is clear that Labour remain deeply divided both north and south of the border.

"We will be taking absolutely nothing for granted however, and will be working hard every day from now to next year's election to retain the support of the people of Scotland - just as we have done every day since 2007.

"Since coming into office over eight years ago, the SNP have been squarely focused on delivering on the issues that matter to the people of Scotland. We have frozen the council tax, protected free higher education, scrapped prescription charges, protected free personal care, maintained the concessionary travel scheme, increased Scotland's health budget to record levels, put over 1,000 more police on our streets, and protected those hardest hit by unfair Tory welfare cuts.

"As First Minister Nicola Sturgeon has outlined, however, there is still more to do and, if re-elected in 2016, the SNP will build on our strong record and deliver further improvements. Our team of 56 MPs at Westminster will also be holding the UK Government to account, continuing to act as the real opposition to the Tories."