Plans to use the next general election as a “de facto referendum” on independence will spark the most important internal SNP debate since the 1997 decision to back devolution, Nicola Sturgeon has told the party’s inner circle.

The First Minister told a meeting of the SNP's ruling national executive committee that a “mature conversation” was needed about the push for Scotland to become independent.

It follows the Supreme Court’s ruling that Holyrood cannot unilaterally hold an independence referendum.

Ms Sturgeon immediately said that the next election of MPs would instead serve as a test of public opinion on the constitution and that she would seek exit negotiations with the Prime Minister if more than 50 per cent of the popular vote went to pro-independence parties.

Party activists will have the opportunity to debate and revise the plans at next year’s spring conference, with some in the SNP’s Westminster group sceptical about the strategy.

READ MORE: Interview: SNP's Flynn hints at doubts over de facto vote

Interviewed by The Herald on Sunday after his election as the leader of the SNP's Westminster group earlier this month, Stephen Flynn said MPs would be "on the frontline" and that many options "have been floated” about the route ahead when asked about the proposal Ms Sturgeon first unveiled to Holyrood in early summer. 

“Of course my colleagues have a lot at stake in that regard. It’s us who will be on the frontline and I have no doubt our voices will be heard in relation to the up coming debate,” he said.

There is concern among some in the SNP over achieving 50 per cent of the vote - a very high bar for a single party in a general election.

In 2015 when the SNP won a record 56 out of 59 seats at the general election the party took 50% of votes. With the Greens the total votes for the two independence supporting parties came to 51.3%.

But it is also unclear how, even if the SNP and other independence supporting parties won more than 50% of the popular vote, how independence  would be achieved if the new UK Government would not negotiate. And if the Yes side lost, would that mean the current independence campaign would then be shelved?

Some in the party want the next Holyrood election in 2026 to instead be regarded as the de facto independence referendum.

In closing remarks to the NEC meeting on December 17, reported in The Times on Thursday, Ms Sturgeon said the forthcoming internal strategy discussion was “the most important one since that 1997 debate which I remember — as some others on the call will — very, very well”.

She said: “It’s of a similar significance and nature to that decision. So it is first and foremost a decision for the party to take. But we are Scotland’s party and I’ve always liked that description of us as well. And this is a decision that matters way beyond our ranks because this is a decision that will determine whether we can progress [the] independence argument to the point we want to take it, winning the case for independence and winning majority support for independence, or whether we set it back.”

The SNP was initially sceptical of Tony Blair’s plan to hold a referendum on setting up a Scottish Parliament when he announced it in 1996 but the following year backed devolution as the party’s gradualists won the argument that it would be a stepping stone to independence.

Another comparison is with the vote in 2000 when Alex Salmond persuaded the party to adopt the mantra that “independence must be achieved by a referendum”.

The SNP had argued that a majority of Scottish seats in either the House of Commons or a devolved Scottish parliament would constitute a mandate for independence negotiations.

Ms Sturgeon’s preference is that the UK Government agree to a referendum and follow the same process which paved the way to the 2014 vote. However successive Conservative Prime Ministers have rejected the suggestion out of hand.

In her address to the senior party officials, Ms Sturgeon said that there was an opportunity to widen the appeal of independence in the wake of the court’s ruling.

She has previously sought to re-brand the independence push as Scotland’s democracy movement.

She said that the internal debate must not “just dismiss options that others may see as attractive because we think that they’re not practical”.

“We’ve got to explain why they’re not practical and take the party along in a mature conversation about this, which is why I’m absolutely not saying that we just rule out particular options.

“But equally in terms of showing leadership, we cannot knowingly lead the party down a road that is not taking us where we want to go but that has to be as part of a proper, open debate.”

She called on the spring conference to be a “springboard” for nationwide campaigning for independence.

Six opinion polls after the court’s decision showed support for independence increasing to the point where it would win a referendum.

The most recent survey, by Savanta for The Scotsman, found a narrow lead for “no”.