BORIS Johnson has given a “guarantee” to block a second independence referendum if he wins a majority on December 12, as his party appealled directly to pro-Union Labour supporters to vote tactically for the Tories.

Mr Johnson makes the commitment in the introduction to the Scottish Conservative manifesto, which carries the words “No to Indyref2” prominently on the cover.

The Prime Minister claimed Scotland had been “paralysed”  and “trapped like a lion in a cage” by Nicola Sturgeon and the SNP refusing to accept the No result of 2014.

HeraldScotland: Camley's Cartoon: PM launches Scottish Conservative manifesto.Camley's Cartoon: PM launches Scottish Conservative manifesto.

He said stopping Indyref2 would “restore confidence and certainty to businesses and families”, with politics returning to priorities such as public services.

He wrote: “If the outcome of this election is a strong Conservative majority Government - then I can guarantee that we will reject any request from the SNP Government to hold an independence referendum.”

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Referring to Ms Sturgeon requesting a Section 30 order to give Holyrood the power to hold a legally watertight referendum, Mr Johnson went on: “There will be no negotiation - we will mark that letter return to sender and be done with it.

“You have already made your decision five years ago, when two millions Scots said no to Independence. Nicola Sturgeon promised you this was a once-in-a-generation decision and I will hold her to that.”

The comments contradict those of his Scottish Secretary Alister Jack, who recently said a SNP majority win in the 2021 Holyrood would be a “democratic mandate” for Indyref2.

Ms Sturgeon has said she wants to hold another referendum in late 2020, but has so far failed to explain what she would do if the UK Government simply refused.

Jeremy Corbyn has said Labour would not permit Indyref2 in the early years of a Labour government, but would relent if pro-independence parties won a majority of seats in Holyrood at the 2021 Scottish election on a clear commitment to another vote on the constitution.

Launching the manifesto in North Queensferry in Fife, he also said it would be a "tragic mistake [to] smash up" the Union.

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Introducing the Prime Minister, acting Scottish Tory leader Jackson Carlaw urged pro-Union Labour supporters to vote Tory to thwart the SNP.

He said: “I say to all pro-Union voters who are thinking hard what to do at this election - voters who might have been thinking of giving it a miss, or people who haven’t voted for us before - forget Scottish Labour. 

“They have abandoned the Union and Jeremy Corbyn has abandoned you. Instead lend us your vote. Join with us. Because this is important.

“Together, let’s make Nicola listen for once and together can move on as a country.”

Mr Carlaw later said Ms Sturgeon should stick to her "once-in-a-generation" promise on Indyref2, defining a generation as around 40 years - the same gap as between the EU referendums of 1975 and 2016.

In his foreword to the manifesto, Mr Carlaw said: “Scotland is facing the real risk of another divisive referendum on independence - within months.

“Nicola Sturgeon has said she wants another referendum next year. Jeremy Corbyn - and countless Labour politicians - have made clear they will not stand in her way.

“If the SNP and Labour win a majority of seats in the UK Parliament between them, a Corbyn-Sturgeon pact is on the cards. 

“The SNP will hand the keys of Number 10 to Mr Corbyn in return for the referendum they long for - with a second referendum thrown in for good measure.

“We now have the chance to reject, once and for all, the SNP’s attempt to drag us back to another referendum on independence.”