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Starmer has signed up Jeremy Corbyn’s former chief of staff in another big boost to his leadership campaign.

The Mirror has learned that the Labour candidate has recruited Simon Fletcher as his top strategic adviser as he bids to win the top job.

Mr Fletcher was the architect of Mr Corbyn’s shock victory in his first Labour leadership contest four years ago.

He went on to become the outgoing leader’s chief-of-staff and later Labour’s campaigns director before quitting in 2017.

The former Ed Miliband and Ken Livingstone aide is widely respected across the party as a strategist and political operator.

His arrival came as Unison, the country’s biggest union, announced it was backing the shadow Brexit secretary for leader.

(Image: Internet Unknown)

Mr Starmer also today became the first candidate to get onto the ballot paper as he secured the support of enough MPs to get over the line.

The Mirror understands that ex-Darlington MP Jenny Chapman, who was Mr Starmer’s deputy on Brexit but lost her seat at the election, will chair his campaign and Kat Fletcher, another ex-Corbyn aide, will become a top campaign official.

Many Labour insiders had believed that Rebecca Long Bailey, the ‘continuity Corbyn’ candidate, would try to recruit Mr Fletcher.

But he said: “There are a number of excellent candidates for the Labour leadership, who would deserve full support if they were elected.

“However, I think Keir Starmer has the greatest capacity to deliver most effectively on the challenges Labour faces."

Mr Fletcher claimed that his new boss was garnering support on the Left of the party, who might have been expected to back Ms Long Bailey.

“On a personal level, I was struck over the Christmas break by just how many friends of mine who consistently supported Jeremy Corbyn were also now supporting Keir Starmer for leader” he said.

“He has set out a refusal to oversteer away from Labour’s radical policy framework, a commitment to seek to unify the party and an intention to be serious about holding Boris Johnson to account, in order to face the electoral challenge ahead.

“For these reasons I will be supporting him in this leadership election.”

After Labour’s devastating election defeat, Mr Fletcher told the Mirror he had “desperately” wanted Mr Corbyn to become Prime Minister.

But he added: “Those of us on the Left of the party who have campaigned for socialist politics must be open and honest about what we need to do to ensure that Labour can come back.

“Jeremy Corbyn has inspired very large numbers of people, including young people. It is one of his greatest legacies.

“But poor satisfaction ratings amongst some other voters are undeniable. We can argue about why that is, but sadly it was a fact.”