LEN McCluskey has said Scottish Labour is on the verge of extinction under new leader Anas Sarwar and described Keir Starmer as “appalling”.

Speaking to The Herald on Sunday, the former trade union chief said the party had still failed to grasp the needs of Scottish voters and tempt them to choose Labour over the SNP, while in Westminster he hit out at Sir Keir’s leadership.

The left-wing trade unionist and former General Secretary of Unite was speaking ahead of a visit to Glasgow on Tuesday to promote his newest book, Always Red.

The book details rifts between Labour factions and incidents which have, in his view, led it to where it is today.

Mr McCluskey said there was “no doubt that Scottish Labour is in decline” under Anas Sarwar, who took over as leader in 2021 after the resignation of Richard Leonard – the candidate supported by McCluskey in his leadership bid of 2017.

He added: “[It] may well be terminal decline unless Anas is able to offer a radical alternative, and grasp an imaginative approach to the constitutional question.”

He reiterated his view that Scottish Labour must support a second independence referendum for Scotland and said he would vote for independence if he had the opportunity.

Acknowledging such a move would be risky for the party, as it would eventually have to choose to support independence or the union, he added: “The danger at the moment is that we are stagnating.

“From what I'm told by my comrades in Scotland, there is no breakthrough coming and therefore, the real danger for me is that Scottish Labour become an irrelevance in Scotland.

“I couldn’t imagine that in my life. Growing up in in Liverpool, Scotland was always an area that we looked to for radicalism, for ideas, for imagination, for passion; all of that seems to have gone.”

While embracing a vote on the constitution, Scottish Labour could also turn its attention to federalism, suggested Mr McCluskey

He said: “I believe Scottish Labour needs to embrace the concept of a second referendum without necessarily saying which way it would stand on that question. Perhaps it's time for imaginative thought about the concept of a federal UK.

“In the Great Northern conurbations in England, the mayors, in particular the likes of Andy Burnham, have challenged the government about distribution of wealth, and I think that could be an area that Scottish Labour pick up in a very positive way because at the moment they are stagnated.”

Mr McCluskey said that the while the SNP used to be known as the ‘Tartan Tories’ they had changed their fortunes thanks to “charismatic leaders like Alex Salmond and Nicola Sturgeon”, adding: “They've stolen Labour’s clothes, they are seen as a more radical social democratic party and meanwhile, Scottish Labour has just sunk almost out of sight.”

A well-known supporter of Jeremy Corbyn, the union chief’s disdain for his successor Sir Keir was hardly disguised, when he said he believed he had done “appallingly” since he took Mr Corbyn’s place.

He said Sir Keir had failed in his aims of uniting the party, with both left and right still at loggerheads, and he no longer trusted him.

He explained: “I had a good relationship with Keir at the outset of his stewardship but having been elected on a radical platform, it is very evident now that he was kidding people - he didn't believe in those 10 values that he ran on.”

During his leadership campaign, Sir Keir set out 10 pledges which he said he stood for, including social and economic justice, strengthening workers’ rights and common ownership.

Mr McCluskey said he believed the Labour leader did not stand for those things, and he had also failed to unite Labour.

He said: “He spoke to me at the time and said his most important thing was to try and unify the party.

“I told him that there would be a desperate wish for everybody to unify around him. Six months later he suspended Jeremy [Corbyn].

"I reached an agreement with him to allow Jeremy to come back and he reneged on the agreement. It was at that point that I lost my trust in Keir, and since then, the party has moved dangerously into an authoritarian arena where debates and discussion have been closed down in the constituency Labour parties.

"I think we're approaching something like 200,000 of our members have left the Labour Party and he continues to attack my wing of the Labour Party.

"At the moment, the idea that that is the way to win back the red wall seats in our English heartland is completely false and wrong.”

Looking back on his time as Unite’s general secretary, Mr McCluskey said one thing still angers him – the Falkirk saga, almost a decade ago, in 2013.

The party launched an investigation after claims of vote rigging in the internal candidate selection process, with it eventually concluding to find no wrongdoing.

It had been claimed that Unite had recruited workers from Grangemouth in to the Labour party and paid their membership fees in order for them to back Karie Murphy, the union’s preferred candidate.

The whole episode led to then-leader Ed Miliband changing the rules so that members of trade unions had to ‘opt in’ to the party, rather than opt out.

Mr McCluskey said he would still accept an apology from Labour today, for the events of 2013, and has yet to receive one.

He added: “It's never too late to apologise for wrongdoing but I won't be holding my breath, let me put it that way.

“Anybody who studies that whole period, I've no doubt that students of politics will do that, they would be amazed at the ineptness, and corruptness, of the Labour Party at national level, a sham invest investigation, Ed Miliband blundering, trying to show how macho he was by taking on the big bad trade unions and big bad Len McCluskey and it was a disgrace.

“Not only did they not apologise, ultimately, they found that there was no wrongdoing.”

His former employer, Unite, is facing another investigation currently, after it spent £98m on a hotel and conference complex in Birmingham, which has been valued at less than £30m.

Mr McCluskey’s successor Sharon Graham has ordered an independent inquiry into the project, which he says will “answer all the questions people have”.

Asked about the probe, he said: “The Birmingham complex is a fabulous project. But there are questions that have been raised.

“An independent inquiry has been ordered by the new general secretary and all those questions will be answered.”

He also said he had “no involvement” with the project, explaining: “During my time as general secretary we refurbished literally dozens of buildings. I didn't have any input into that whatsoever.

"My finance director had complete authority, but I'm pretty confident that there was no wrongdoing. That's why the inquiry is looking into it.”