A CONSERVATIVE attempt to insert a “union guarantee” into Holyrood’s alternative Brexit Bill has backfired by uniting Scottish Labour and the SNP against it.

The Tories have proposed a sweeping amendment to the EU Continuity Bill which would prevent Scottish ministers doing anything to “impede or obstruct” the UK single market after Brexit.

It would force Scotland to stay in lockstep with the rest of the UK on all "commercial, economic, social and cultural activity".

MSP Adam Tomkins challenged the SNP, Labour and LibDems to support the change.

However it prompted a scathing response from Labour, who accused Tory leader Ruth Davidson of a trying to “put a sticking plaster over her party's attack on devolution”.

With the Greens already backing the Continuity Bill, it suggests there will be a cross-party consensus if the SNP legislation goes to a final vote on Wednesday.

The Bill could yet be withdrawn if the UK and Scottish governments break the stalemate on the distribution of devolved EU law after Brexit, but the odds of that now appear slim.

If passed, the UK government is expected to challenge the Bill at the UK Supreme Court, something which would be unprecedented in devolution.

Mr Tomkins said: “The best way forward is still for the SNP to back UK-wide legislation.

“But if the SNP persists with its proposals, at least this amendment would deliver a ‘Union Guarantee’, making it clear that Scotland will not diverge from the rest of the United Kingdom in key areas such as regulation and case law.

“Wherever parties stand on the constitution, all should be able to support giving this guarantee. It is in Scotland’s interests we do so.”

However a Scottish Labour spokesperson: "The UK single market is the key market for Scottish businesses and any attempt to threaten it would do untold damage to our economy.

"But this attempt by Ruth Davidson to put a sticking plaster over her party's attack on devolution is jaw-droppingly shameless.

"It is the Tories who have caused the whole disastrous Brexit situation - and their shambolic handling of the negotiations continue to make a bad situation worse.

"Their disregard of devolution at Westminster is causing further frustration and exasperation.

"The Tories should be protecting devolution and not playing games at Holyrood."

A spokesperson for SNP Brexit Minister Michael Russell added: “This is yet another stealth attack by the Tories on devolution - if they can't take Scotland's powers away, they want to prevent us using them.

“This amendment simply confirms that Ruth Davidson's Tories care more about the constitution than protecting jobs, businesses and livelihoods here in Scotland."

The SNP's Europe spokesman will today say Scotland's long history of political, commercial and cultural engagement with Europe could let it act as a "bridge" between post-Brexit UK and the EU.

MP Stephens Gethins will tell the "Brexit: Should I Stay or Should I Go?" conference in London: "Scotland is well placed to act as a bridge between the UK and the rest of Europe, regardless of our future constitutional status, and regardless of the final outcome of the negotiations. The work to build that bridge starts now.”