THE leader of the Scottish trade union movement claims a second independence referendum is “inevitable” if Theresa May insists on imposing a hard Brexit on Scotland.

Grahame Smith warned that the Prime Minister vetoing Nicola Sturgeon’s proposals to keep Scotland in the single market would lead to a fresh vote on independence.

Smith, the general secretary of the Scottish Trades Union Congress (STUC), said Westminster should not block a referendum if Holyrood, where there is a pro-independence majority, votes for it.

His stance is at odds with Scottish Labour leader Kezia Dugdale who has said she will "never" support one.

The STUC is not affiliated to Labour, but a large number of its member unions such as Unite, Unison, the GMB, the CWU, Usdaw and Community are in the party.

Smith, a longstanding Labour Party member, made the remarks in an article in the forthcoming edition of Scottish Left Review.

His intervention will be interpreted as a call to Scottish Labour not to oppose a second referendum, if the pro-independence parties at Holyrood, the SNP and Greens back one.

Dugdale used Scottish Labour's conference to launch a new online initiative where Scots can pledge their opposition to a second referendum.

However, Smith said there would be "inescapable background noise" about a new referendum at the STUC annual congress next month in Aviemore, at which Sturgeon and Dugdale are both to deliver speeches.

Smith's declaration that a second independence will come as a blow to Dugdale and a boost to Sturgeon, who will be keen to court the support of union members at the STUC event from April 24-26.

Speaking about the prospect of a hard Brexit, Smith said: "Does all of this make a second referendum on Scottish independence more likely? While this is not part of the formal congress agenda, it is inescapable background noise."

Smith said Sturgeon had been right to demand powers be devolved over areas such as employment rights, business regulation and immigration, aimed at allowing Scotland to remain in the single market even if the rest of the UK leaves the bloc.

However, he said if May rejects the plan when Article 50, the formal process for leaving the EU is triggered this month, a second independence referendum was likely to take place.

He said: "The Scottish Government is right to attempt to ensure that the democratic wishes of the people of Scotland in relation to the EU are respected.

"Its differentiated option is a compromise which would see Scotland, in effect, abandon the cause of independence in favour of remaining in the EU.

"It’s an approach not without economic, political, legal and diplomatic complications and would require substantial new powers, including over employment and union rights, to be devolved to Scotland."

He added: "If the UK Government is resistant to this option, another independence referendum is inevitable.

And, if there is a majority in the Scottish Parliament in favour of such a vote, it should not be blocked by Westminster."

The STUC did not adopt a position for or against independence ahead of the 2014 referendum.

In her speech to Scottish Labour conference last month, Dugdale said: “I do not want another referendum on breaking up the UK. Our country still bears the scars of the last one, and no one wants to go through that again any time soon.

“That’s why Labour will never support one in the Scottish Parliament."