A SECOND referendum on independence could take place by 2021, Nicola Sturgeon has confirmed, with a list of 'triggers' to be included in the SNP’s Holyrood manifesto.

In an interview with the Sunday Herald to mark the first anniversary of the historic vote, the First Minister said her party’s manifesto would set out “appropriate” timescales for a new ballot.

During the general election campaign, Sturgeon said there would have to be a “material change” in circumstances before she would go to the people again on independence.

The only concrete example she cited was the UK leaving the EU in David Cameron’s in-out referendum against the wishes of most voters in Scotland.

However she told the Sunday Herald her platform for 2016 would include far more detail on possible triggers, as voters rightly expect clarity on the issue.

She said: “Our manifesto will set out what we think around the circumstances in which, and the possible timescales in which, a second referendum might be appropriate.

“It will then be down to people to accept whether they vote for that manifesto.

“People, as they vote in the election next year, will know what our position is and what our view is on the circumstances in which a second referendum might be appropriate.”

The comments show that, despite calling the 2014 referendum a once in a generation event, Sturgeon is now open to holding another plebiscite in the next parliament, which runs to 2021.

Sources close to the First Minister said any trigger list would be limited, but could include a general option allowing for a referendum in the event of a sudden “black swan” event, a dramatic and unforeseeable change such as Cameron embarking on an illegal war. The full position is expected to be laid out later this year.

However, with the SNP predicted to increase its majority at Holyrood in May, the idea of a ‘mibbes aye, mibbes naw’ position on a second referendum may not be enough satisfy the clamour for a new ballot from within the SNP, its ranks swollen by tens of thousands of impatient Yes supporters.

The party’s annual conference in Aberdeen next month will be a key pointer to its mood.

The FM’s allies also admit the appropriate circumstances may not arise before 2021, and in her interview Sturgeon said winning support for independence remained a challenge.

She said people were “kidding themselves” if they thought simply changing the SNP’s position on Scotland's currency was a shortcut to victory, and said the Yes side had to work to overcome people’s sense of “uncertainty” about leaving the UK.

She also said she would like the polls to show independence was the settled will of the Scottish people before an “emphatic” and “decisive” Yes vote.

Although two recent polls found a narrow majority for independence, a YouGov survey yesterday found 52 per cent of people would vote No in a ballot tomorrow.

Opposition parties are likely to focus on whether Sturgeon’s triggers represent genuine watersheds in UK politics, or are so likely they make a second referendum inevitable.

Alex Salmond yesterday said the renewal of Trident should be a trigger for 'Indyref2'.

However Westminster voted in 2007 to “maintain the strategic nuclear deterrent beyond the life of the existing system”, and next year’s so-called Main Gate decision is actually about whether to commission three new nuclear submarines or four.

Salmond has also suggested a failure to deliver the pre-referendum Vow on more devolution, an EU exit and “austerity to the max” could trigger a second plebiscite.

But Salmond's list was not endorsed by Sturgeon.

A source close to the First Minister stressed no final decision had yet been made on specifics.

Scottish Tory leader Ruth Davidson added: “Most people in Scotland want to put last year's referendum behind them and get on with life. Despite her own promises, it is now clear that Nicola Sturgeon wants to take Scotland back to a neverendum. The SNP has shown yet again that it will always put its obsession with separation first.?”

Labour MP Ian Murray said: “We were told the referendum was a once in a generation opportunity, but now Nicola Sturgeon appears to be going back on her word and is following her members rather than doing what is in the best interests of the country. The SNP want the election to be about past arguments because they can't defend their record in government.”

Scottish LibDem leader Willie Rennie accused Sturgeon of “putting her party before the country by plunging us into another protracted campaign for independence”.

He said: "With the police in crisis, the NHS in difficulty and educational standards slipping, we need a government focussed on the challenges in our daily lives rather than their own political ideology."