NICOLA Sturgeon and Theresa May are on a collision course over independence after taking up seemingly irreconcilable positions on a second referendum.

After Ms Sturgeon said it was “inconceivable” that Westminster would block a vote, Downing Street said the 2014 result was “decisive” and it was “vital” it was respected for a generation.

Former SNP justice secretary Kenny MacAskill warned Mrs May might block a referendum, saying she could simply tell the First Minister: “No, you’re not getting it.”

The stand-off follows Ms Sturgeon saying on Thursday that she would publish a draft referendum bill for consultation next week, and warning Mrs May she would go to the country unless Holyrood secures sweeping new powers as part of a deal on Brexit.

However the bill depends on Westminster devolving temporary powers to Holyrood to make the referendum legally watertight, as happened before the 2014 vote.

Scottish Tories’ leader Ruth Davidson has warned refusal would provoke a backlash and boost the independence case, as happened in Catalonia when Madrid blocked a vote on autonomy.

But the Prime Minister’s official spokeswoman last night sent the strongest signal yet that Westminster could refuse regardless.

She said: “There was a referendum in 2014 that addressed this issue that was legal and fair. The result was decisive and both parties agreed at the time to respect it. It was a referendum that was once in a generation and it’s vital it should be respected.”

A generation is commonly defined as around 20 years.

Ms Sturgeon will take a defiant stance in her closing speech to the SNP conference in Glasgow today by setting out a four-point plan to expand Scotland’s trade in Europe, including permanent representation in Berlin and a doubling of trade and investment staff in the EU.

She will say: “Make no mistake, the growth of our economy right now is threatened not just by the prospect of losing our place in the single market, disastrous though that would be.

“It is also the deeply damaging, and utterly shameful, message that the Tories’ rhetoric about foreign workers is sending. More than ever, we need to tell our European friends that Scotland is open for business.”

She will also say Brexit has created “a new political era and a new battle of ideas” and the “primary contest of ideas in our country is now between the SNP and the hard-right Tories”.

She told Radio 4 yesterday that Scotland found itself in a position its people never wanted with Brexit, and Westminster could not justify refusing a new vote on leaving the UK.

She said: “If there is a view in the Scottish Parliament that the best way to protect our interests is to offer the choice of independence again, the idea that the same party that put us into that position would then deny us that choice I just find inconceivable.”

But she was later contradicted by Mr MacAskill, who told the IdeaSpace conference in Glasgow: “We face challenges in an independence referendum. It’s not just a matter that the Scottish Parliament says, ‘We’re going to have a bill for a Scottish referendum, off we go.’ “It may be perfectly feasible that you could face a Tory government that will replicate Spain and say ‘No, you’re not getting it. We don’t care about the clamour, we’re not giving you it’.

“Catalonia have demanded it, insisted upon it, marched more than we have marched, and they’ve still not been granted a referendum from Prime Minister Rajoy.”

“Can we be guaranteed that Theresa May will just say ‘That’s fine, on you go?’”

Labour external affairs spokesman Lewis Macdonald said: “Scotland deserves better than two nationalist governments posturing at each other.

“Hard Brexit supporting Tories or independence supporting Nationalists are simply advocating their own form of separation, not a future where we work together.”

Reacting to the SNP trade plans, Tory MSP John Lamont said: “You don’t show Scotland is open for business by threatening to pull us out of our own Union and creating more instability.”