SCOTLAND'S trade union movement has backed the country's right to hold a second independence referendum should the controversial power grab bill be passed in Westminster against Holyrood's wishes.

The stance taken by the Scottish Trades Union Congress (STUC) emerged in a briefing sent by the organisation to MSPs on whether Holyrood should consent to the legislation.

It said that should the Scottish Parliament refuse to back the bill and the UK Government pressed on with it, that "makes the case for a second independence referendum unanswerable."

The STUC briefing set out why it was opposed to the bill which as well breaking international law also aims to create a common standard of rules across the UK once powers return from the EU to Westminster.

Its measures have already been criticised by farmers, education bodies, parliament committees and academics as a risk to devolution and food standards.

The STUC note was shared by the Scottish Greens MSP Ross Greer on social media last night.

"The STUC is strongly opposed to the internal market bill which would limit the ability of the Scottish Parliament to influence public policy and represents an attack on devolution," it said.

"Specifically the STUC's concern mainly relates to state aid, the market access commitment and intergovernmental relations. Should the UK Government proceed with the bill against the wishes of the Scottish Parliament it makes the case for a second independence referendum unanswerable."

Greer tweeted: "Today the Scottish TUV voice of half a million trade unionists in Scotland once again defended this country's right to self-determination."

The Scottish Government confirmed last week that it will recommend Holyrood refuse consent for legislation. In a debate and vote today MSPs are expected to deny consent.

Ahead of the vote  Scotland's Constitutional Affairs Secretary Michael Russell said: “This is a defining moment that will determine both the future of the Scottish Parliament and whether or not the UK can be described as a partnership of equal nations.”

UK Government ministers have already accepted the controversial Bill breaches international law, and Russell added it would be “equally outrageous if they decided also to break the constitutional convention that the Westminster Parliament does not legislate in devolved areas without the consent of the Scottish Parliament”.

Scottish Secretary Alister Jack has already said the UK Government plans to press ahead with the legislation – without the backing of the Scottish Parliament if necessary.

Russell added: “The UK’s established constitutional rules mean that the consent of the Scottish Parliament is required for the UK Government’s Internal Market Bill to proceed.

“If the Parliament refuses to grant consent then that should kill the Bill stone dead. It will demonstrate beyond all doubt that the UK Government does not believe the UK to be a partnership of equals.”

The STUC's position will pile pressure on Scottish Labour to reverse its opposition to a second independence referendum.

In January Grahame Smith, the former general secretary of the Scottish Trades Union Congress (STUC), said the SNP's landslide victory in the General Election the previous month made it clear that voters wanted a referendum.

Smith, who retired in the spring after 14 years in the post, said the election result presented Labour with a dilemma but also an opportunity.

Writing in the magazine Scottish Left Review, he said: “The first minister has made it clear that she will now push for indyref2. This has left Labour in a quandary: it cannot hold, as it has, that the overall election result gives the Tories a Brexit mandate, and simultaneously maintain the result in Scotland cannot be viewed as a mandate for indyref2.

“The democratic wishes of the people of Scotland need to be acknowledged. The Scottish Labour movement should support indyref2.”

Scottish Labour leader Richard Leonard promised a review of its position after Labour was left with just one Scottish Westminster seat after winning its lowest general election vote share in Scotland in the modern era, at 18.6%.

Leonard was forced last summer into accepting that the party could support a fresh referendum after John McDonnell, the shadow chancellor, unexpectedly announced that policy.

But in June rather than adopt a position more open to a new vote Leonard's adopted a more hardline opposition to one.

Critics said he was out of touch with the mood among Scots amid polls showing support for independence at up to 55%.