A key ally of Jeremy Corbyn has said failing to elect Rebecca Long-Bailey to be the Labour party’s next leader risks turning the clock back to 2015 and the leadership of Ed Miliband.

Mark Serwotka, head of the civil servants’ union PCS, said that the other candidates – Keir Starmer, Lisa Nandy and Emily Thornberry – would either struggle to maintain the radical policies of Jeremy Corbyn or have failed to realise that prevarication over the party’s Brexit policy was a key reason for the devastating 2019 defeat.

“The issue for me is that some want to go back to 2015 and Ed Miliband and lose the party’s radical anti-establishment, socialist message,” he said.

Blaming the party’s gradual shift towards supporting a second referendum for the loss of traditional Labour seats, Serwotka said Long-Bailey uniquely understood why this happened and would ensure that a similar backing away did not happen again.

“I believe that Rebecca Long-Bailey is the person who understands the defeat and the need to still argue for a progressive radical manifesto.

“The other candidates either don’t support a radical manifesto, or if they say they do, would find it hard to square the circle on Brexit because I am not sure they really accept the analysis that I have and that Rebecca has,” he said. He is also backing Richard Burgon to be deputy.

Rather than blame Corbyn for the defeat, Serwotka said “the whole Labour movement was complicit” in allowing the party to drift away from a settled position on Brexit.

Serwotka, who was elected as the union’s general secretary in 2000 and re-joined the Labour party in 2016 after being expelled more than two decades earlier, said anti-Corbyn MPs must accept some responsibility for the defeat and backed open selections in future.

“If you are a Labour party member and you see your MP slagging off the leader every day, what are you supposed to make of that? I know what I think of that. I would rather have a different person standing,” he said.

In line with other civil service unions, PCS will not officially back any candidate and does not affiliate to Labour.

The union is currently embroiled in several disputes in Whitehall including a month-long strike at the Foreign Office.

Following demands by Boris Johnson’s senior adviser Dominic Cummings for “weirdos and misfits with odd skills” to apply for jobs directly to him in Number 10, Serwotka wrote to the cabinet secretary, Mark Sedwill, demanding to know if the government had torn up Whitehall’s employment practices.

“I am seeking your assurance that the UK civil service employment practices and standards will not be undermined, that PCS members’ employment rights will be maintained and that any changes will be subject to consultation with the unions,” he wrote on Wednesday.