UNIONIST fears have been raised that the SNP Government will seek to hold the next but one Holyrood election in 2019 - two years earlier than expected - from which to launch a bid for a second independence referendum that summer.

A second poll in just four years' time could provide the optimum opportunity to deliver a Yes vote for Scotland breaking away from the UK with the belief that: the Nationalists would still be riding high; Labour would continue to be in a weakened state and a Holyrood election in 2019 would give Scotland the chance to avoid a third-term Tory government in London as the next General Election falls in 2020.

One senior Unionist source at Westminster suggested that, now the power over elections in Scotland was being transferred to Holyrood, the Nationalists could seek to "gerrymander" the electoral timetable to its own political advantage.

"People here are beginning to think they might try to gerrymander the process and propose elections in 2019 and then in 2023," he explained, adding: "But this will be seen for what it is; a shameless political exercise to use a 2019 election as a launch-pad for another independence poll."

With the SNP clear favourites to return to power after next May's Scottish election, some within the party believe Nicola Sturgeon should opt for the earlier date rather than stay in power for five years.

Duncan Ross, a former national secretary of the SNP, did not make reference to a second poll but said: "I can see the argument for 2021. If the Tories win again in 2020, people will think the Tories keep winning, what's the point with Westminster? But for me it would make sense to go in 2019."

He explained: "Labour will not have figured themselves out in Scotland by then, they will still be in a mess, and the Tories will be in the midst of their austerity programme.

"We have a real opportunity there. If we go three years to 2019 we'll have a bigger chance of winning another four years in power after that and then we are talking about building a long trajectory for an SNP government and that seems very attractive."

Mr Ross admitted the next Scottish Government would face difficult decisions after 2016 but he added: "We'd want to be going to the country while other parties are in a difficult place."

A spokeswoman for the SNP dismissed talk of a second referendum, saying: "From Better Together last year, Labour, the Tories and Lib Dems - with just one seat each in Scotland - seem to be delusional together now. The Scottish Parliament operates on a fixed-term basis.

"The No campaign parties are the only people talking about another referendum when they should be working on behalf of people in Scotland.

"The priority of the SNP's 56 MPs will continue to be standing up for Scotland at Westminster," she added.

The First Minister has thus far sought to downplay the issue of a second independence referendum after last September when the No vote won by 55 per cent to 45 per cent.

She has made clear there would have to be a "material change" to bring about the circumstances for another referendum, citing the only example, thus far, of the UK having voted to leave the EU in the forthcoming poll on Britain's membership while Scotland voted to stay in.

Expectations are high that Ms Sturgeon will place a form of words in the 2016 Holyrood manifesto, leaving the option of calling a second poll open and satisfying all the SNP's new members.

But she and her colleagues believe the impetus for another referendum must come from the Scottish voters.