SCOTLAND'S most powerful union leader has attacked Labour's “complete lack of leadership” over Brexit.

Scottish Trades Union Congress general secretary Grahame Smith hit out at Richard Leonard and Jeremy Corbyn's “lukewarm attitude" to the EU.

Corbyn and Leonard "must get their act together", said Smith.

Smith, a Labour party member, said they should at the “very least” back remaining in the single market for the UK. However, Smith said his "personal position" was to hold a second referendum on the EU.

The intervention comes ahead of a rally Corbyn and Leonard are to appear at in Midlothian tomorrow. Smith's remarks will be a embarrassment to Leonard and Corbyn, even though the STUC is not formally affiliated to Labour. The STUC has more than half a million members. Labour's major unions affiliates, Unite, Unison and the GMB are also part of the STUC.

Smith said only Nicola Sturgeon and Welsh First Minister Carwyn Jones had shown real leadership in opposing Brexit. Smith, however, did make the point that he remained a supporter of Leonard and Corbyn's general political direction.

He praised Labour's 2017 election manifesto as “spot on” over issues of workers' rights, public ownership and tackling equality.

However, he said Leonard and Corbyn were "fundamentally wrong" about the EU.

Leonard has mirrored the position held by Corbyn of honouring the outcome of the 2016 referendum, but backing a deal that allows single market access.

Both Scottish and UK Labour have refused to support full membership of the single market and customs union.

Sturgeon has previously said the position is "effectively backing the Tories’ extreme Brexit".

Smith said Labour at Holyrood and Westminster was letting the Tories “away with it” over their plans. Speaking to the Sunday Herald, Smith said: "We are sleepwalking towards disaster, with a significant threat to living standards, jobs and employment rights.

"The Prime Minister is utterly incapable of providing leadership because she is paralysed by a fear of extreme Brexiteers. There is complete and utter lack of clarity over what Brexit will mean. The UK Government is being allowed to get away with it."

He added: "The Labour leadership has got to get its act together and ditch its longstanding and lukewarm attitude to the EU. At the very minimum they should campaign to remain in the single market. My personal view is that they should get behind the campaign for a second referendum.

"That's the only way to prevent disaster for the country for decades to come. The extreme Brexiteers in the Tory party are happy to have a situation that's fudged. But they are being let away with it. The leadership of the Labour Party is not calling that out.

"The general political direction of Labour over other policies is spot on such as the General Election manifesto. But I believe they are fundamentally wrong on the EU."

Smith said Scottish Tory leader Ruth Davidson had also failed to use her influence to oppose a hard Brexit.

He added: "If Ruth Davidson is the force in the Tory party that a lot of people think she is then she needs to step up and confront the extreme Brexiteers, who are hiding behind the Prime Minister's extreme failure."

Scottish Green Europe spokesman Ross Greer MSP welcomed the intervention from Smith. Greer said: “If we had the support of Jeremy Corbyn and Richard Leonard in keeping the UK in the Single Market and Customs Union it would be very welcome indeed and I hope they both listen to this call from the trade union movement in Scotland."

SNP MSP Joan McAlpine added: "Grahame Smith speaks for Scottish workers and he is absolutely right to warn of the impending disaster of a hard brexit for their jobs - Corbyn and Leonard don't seem interested in listening to the voice of Scottish workers however."

However, a Labour spokesman said Leonard and Corbyn backed a Brexit deal that would protect jobs and living standards. The Labour spokesman said: "Labour are getting on with fighting and holding to account the shambolic Tories' bargain basement Brexit negotiations. The reality is that only the next Labour government will deliver the investment our stagnating economy and working people desperately need and only Labour will negotiate a jobs-first Brexit that retains the benefits of the single market whilst protecting our economy at the same time.”