A second independence referendum should not be held before 2020, warn senior SNP sources, because there is still too much uncertainty that the public would support Scotland leaving the United Kingdom.

Pressure is mounting on Nicola Sturgeon to give a clear indication about the prospect of a rerun poll following last September’s vote when the cause of independence was defeated by 55 per cent to 45 per cent. The subject is a potential flashpoint for the SNP leader at this autumn’s party conference.

Last month, the First Minister said she was “not planning another referendum at the moment”. The only “material change” she has referred to, which would trigger a second poll, is if in the forthcoming EU referendum the UK voted No to continued membership but Scotland voted Yes.

Yet Ms Sturgeon knows that by spring, she will have to decide whether or not to include a reference to a second referendum in the SNP manifesto for the 2016 Scottish parliamentary elections and, crucially, how strong the commitment will be.

The dramatic victory at the General Election when the Nationalists won 56 out of 59 Scottish seats, the prospect of them winning most if not all 73 constituency seats at next May’s Holyrood election and the latest poll giving the SNP a rating of 62 per cent have led some Nationalists to believe that a second referendum must be held soon, and certainly by the end of the decade, to capitalise on the political momentum.

Earlier this month, Alex Salmond, the former party leader, declared confidently that a second referendum was now “inevitable” but that the timing had yet to be determined.

Pete Wishart, the SNP’s Shadow Commons Leader, agreed there would be another poll but noted: “The process of getting there is still uncertain."

Angus Brendan MacNeil, the MP for Na h-Eileanan an Iar, was less equivocal about the timing. When asked if there would be another referendum before 2020, he said without hesitation: “Yes.”

However, there are more cautious Nationalist voices.

One SNP MP said: "We should not rush into a second referendum; it cannot happen before 2020; for goodness sake, we haven't yet properly discussed why we lost the last one.”

The senior party figure recognised, given the surge in party membership to beyond 100,000, that some hardliners would like another vote on independence tomorrow. “But,” the MP insisted, “we have to be sensible; Nicola and the leadership get that.

“We can't afford to lose another one or the cause really will be lost for a generation," added the senior source.

A colleague at Westminster also urged caution, saying: "People have to realise it would be a mistake to directly equate support for the party with support for independence. We have to go when we are absolutely sure the public is with us on this. We're not there yet and probably won’t be by the end of this parliament; it might take a bit longer."

It is thought the plunge in the oil price – still below $50 a barrel - might give some Scottish voters pause for thought in backing independence.

Last week, the Fiscal Affairs Scotland think-tank said the Scottish Government’s latest oil forecasts showed the nation would be more than £7bn a year worse off - £1,300 per head by 2019/2020 - if it became financially independent.

At Westminster, David Cameron has made clear he sees “no need” for another independence referendum, pointing out, like others, that both Ms Sturgeon and Mr Salmond stressed barely a year ago the 2014 referendum was a “once in a generation” or a “once in a lifetime” opportunity.

Also, within the next six months Ms Sturgeon has to decide whether or not the next Scottish parliamentary term will be three or five years to ensure there is no clash with the General Election in 2020.

One theory is that she might opt for 2019, enabling her to fudge a promise on holding a second referendum in the SNP’s 2016 manifesto but with a pointer that the intention is to include a firm commitment to one in a 2019 manifesto.