McCluskey, one of the most prominent backers of the current Labour leader, Jeremy Corbyn, is understood to have expressed admiration for the shadow foreign secretary, although sources stressed Corbyn was secure at the top of the party.
One senior union source confirmed to the Guardian that McCluskey tipped Thornberry to be the next Labour leader whenever a vacancy might arise. The source said the union boss had told friends he felt strongly the next leader should be a woman, given that no woman has ever permanently led the party.
A Unite spokesman declined to comment about Thornberry and said that the Unite general secretary was focused on supporting the party’s current leader at this time. He said: “Len McCluskey backs one leader of our party and one leader only and that is Jeremy Corbyn. Unite’s every focus is on getting a Corbyn-led government into No 10 and seeing the end of this miserable Conservative administration.”
Senior Labour sources added that McCluskey had been musing about potential successors to Corbyn in the run-up to the general election, when allies of the Labour leader feared he would face pressure to resign in the event of a poor performance at the polls. In May, McCluskey, whose union is Labour’s biggest donor, said that if Labour won just 200 seats that could be seen as a “success”. The party’s election tally was 262.
After the party’s better-than-expected performance on 8 June, Corbyn’s backers now believe there is time for other potential candidates – including Rebecca Long-Bailey, a close ally of the shadow chancellor, John McDonnell, as well as Angela Rayner, the shadow education secretary – to build up more experience.
Thornberry, who is also shadow first secretary of state, has impressed Labour members while standing in for Corbyn at prime minister’s questions, as well as McCluskey, whose admiration for her was first reported by the Huffington Post.
She has also won favour for her blunt attacks on Conservative cabinet ministers. She told the defence secretary, Michael Fallon, he was talking “bollocks” during the election campaign.
Corbyn allies believe there is now less urgency attached to proposed changes to Labour leadership rules to ensure a leftwinger could make it on to the ballot paper.
Thornberry and Corbyn have historically had a close relationship as they represent the neighbouring north London constituencies of Islington South and Islington North respectively.
However, Thornberry does not have a long association with the left of the party within which cabinet colleagues such as Corbyn, McDonnell and the shadow home secretary, Diane Abbott, first formed close bonds.
Senior party sources have repeatedly quashed separate rumours that Thornberry is being lined up to replace Tom Watson as deputy leader or to be appointed as a second deputy.
Any change of deputy would require a vote of no confidence in Watson or a successful challenge, which the parliamentary party would be highly unlikely to support. Allies of Thornberry have vehemently denied she is planning any challenge.
Any additional deputy leader’s post would need to be approved by the party conference, but no motion has been put forward by constituency parties or put before the party’s national executive committee.
This article was written by Jessica Elgot and Heather Stewart, for theguardian.com on Thursday 24th August 2017 14.04 Europe/Londonguardian.co.uk © Guardian News and Media Limited 2010