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A major union boss is backing shadow foreign secretary Emily Thornberry to be the next leader of the Labour Party, it emerged today.

Unite’s Len McCluskey is understood to have said the next leader after Jeremy Corbyn has to be a woman and the London MP is a “unifying figure”.

Ms Thornberry is a close ally of Mr Corbyn and was a major figure during the party’s general election campaign. She has also stood in for him at Prime Minister’s Questions.

A source told Huff Post that while there is no vacancy for Labour leader, Mr McCluskey has clear admiration for Ms Thornberry because she is tough and not afraid of taking big decisions.

A spokesperson for Unite - which donated more than £2.4 million to Mr Corbyn’s General Election campaign - dismissed the reports as “gossip” but failed to rule out backing for Ms Thornberry.

He said: “Len McCluskey backs one leader of our paty and one leader only and that is Jeremy Corbyn.”

Labour’s most senior female politician Harriet Harman has claimed she could have led the party in 2010 but was never considered “leadership material”.

The former deputy leader who has inspired dozens of female MPs into politics said she would have beaten Ed Miliband at the 2010 leadership election because the party knew and liked her.

She told BBC Radio 4’s Reflections programme: “There was no sense anywhere, as there often is with women, that I was leadership material.”

Ms Harman, a veteran women’s rights campaigner, has the longest continuous service of any female MP at 34 years. Labour has never had a female party leader.