NICOLA Sturgeon will consider calling a second independence referendum even if the UK Government negotiates a "soft Brexit" and meets her demand of retaining membership of the European single market.

The SNP leader has launched an appeal for sympathetic Tory ministers at Westminster to join her in a "coalition" to advocate maintaining the UK's place in the free trade area as part of a new deal with Europe, an option she said would prove the "least worst" realistically available.

The move led to claims from Labour that the First Minister was "shifting the goalposts" over Brexit by suggesting that securing ongoing membership of the single market, rather than retaining full benefits of EU membership, would be enough to convince her to call off a second independence referendum.

However, a source close to the First Minister denied that the emphasis on trade meant other demands on an ambitious wishlist, unveiled before two opinion polls suggested a short-lived rise in support for independence following the Brexit vote had ebbed away, had been dropped.

Ms Sturgeon has previously pinpointed maintaining membership of the single market as just one priority in light of Scotland's vote in favour of continued EU membership, also citing "key interests" that include safeguarding free movement of people across Europe and employment rights. Even more ambitiously, she has said that Scotland should retain a role in shaping EU rules even after Brexit, meaning even a Norway-style deal which sees the country enjoy European Economic Area status would not be enough to satisfy the SNP.

The source insisted the five tests remained in place while another senior SNP insider admitted that while quitting the European single market would make another vote to leave the UK a near-certainty, retaining membership alone would not be enough to guarantee that Ms Sturgeon would take the independence option off the table, leaving her substantial room for manoeuvre.

Speaking to the BBC, the First Minister said warned that quitting the single market would have "long-term, deep and permanent damaging effects" to the economy and questioned whether the Prime Minister had a mandate to quit the partnership, as some Tory MPs would like.

She said: "I respect that Theresa May has a mandate in England and Wales to remove the UK from the European Union... I think it is much more questionable that she has a clear mandate to take the UK out of the single market. Both in the remain campaign and the leave campaign key figures made the case that voting to leave the EU did not automatically mean leaving the single market."

She added: "Can there be a coalition across the UK that gets the UK into a more sensible position? I think that's worth a good try because Theresa May, she was on the Remain side, so presumably she knows the real risks of removal from the single market."

In response, Downing Street suggested that Ms May would consider Ms Sturgeon’s offer of a soft Brexit UK coalition but made clear she believed a second independence poll was not necessary.

Asked about the coalition suggestion, a spokesman said: "All things are being looked at. The future relationship Britain has with the EU is going to be a hugely important relationship. It’s right that we take our time to consider what the best options are. All areas are being examined to make sure that we have the best possible deal we can get."

Ms Sturgeon has said repeatedly that another independence referendum remains "highly likely" but has also pledged to play a full part in negotiations over the UK's post-Brexit deal, appointing a new minister, Mike Russell, to represent the Scottish Government in talks. While polls conducted in the wake of the Brexit vote suggested a narrow majority of Scots favoured independence, two subsequent YouGov studies have shown near-identical levels of support as were found when the SNP lost the September 2014 referendum.

Scottish Labour said Ms Sturgeon had "changed her tune" over Brexit. Lewis Macdonald, the party's Europe spokesman, said: "This is a definite shifting of the goalposts from the First Minister, who initially said that maintaining our full membership of the EU would be the only acceptable outcome, now it is only about access to the single market."