LABOUR must unite behind Jeremy Corbyn or risk letting the Tories off the hook, trade union leaders representing over a million workers said yesterday.

Their “unequivocal support” to the Labour leader comes as Blair-era has-beens continued to brief against their new chief.

Former Chancellor Alistair Darling told Radio 4’s Today programme he was “not actually sure what [Mr Corbyn stands] for.”

And Labour’s London mayoral candidate Sadiq Khan said he would oppose the rise in corporation tax mooted by new shadow chancellor John McDonnell on Thursday night’s Question Time.

But their interventions came as a survey of grassroots party activists said there was widespread support for Mr Corbyn to press ahead with plans to withdraw Labour’s support for Trident.

Fifty-three per cent were opposed to replacement, the online poll for Labour List found.

Trade unions have tabled motions on rail renationalisation, public services, the Trade Union Bill and the EU to Labour Party conference — and constituency parties are likely to add Trident, housing and welfare to the list.

Under Tony Blair, Gordon Brown and Ed Miliband’s leaderships, commitments made in conference resolutions were routinely ignored but Mr Corbyn is expected to use them to show his errant parliamentary colleagues that his policies have widespread support in the party.

It is thought Mr Corbyn is waiting for such endorsements before making major announcements on policy shifts.

But Mr Darling called on the new leader to “set out with greater clarity what he wants to achieve and how he’s going to do that.”

And he warned: “He must know first impressions gained by the electorate, especially of a party leader, tend not to go away.”

But the leaders of the nine unions in the Trade Union Co-ordinating Group praised Mr Corbyn as “a campaigning popular leader not cut off in Westminster but engaging with the millions of people who need our support.”

The letter — backed by bakers (BFAWU), firefighters (FBU), probation officers (Napo), journalists (NUJ), teachers (NUT), prison officers (POA) and transport workers (RMT and URTU) — condemned the “vicious attacks being launched on Jeremy and his team at the behest of rich media barons.”

“We call on the wider labour movement to unite in defence of the new Corbyn leadership and ensure that we do everything possible to mobilise our full support in taking the attack to the Tories,” the general secretaries wrote.

Only one of the signatory unions, BFAWU, is affiliated to the Labour Party, suggesting Mr Corbyn is in an unprecedented position to harness the campaigning forces of the labour movement.

Mr McDonnell dominated the discussion on Thursday night’s BBC Question Time, delivering heartfelt apologies for previous comments and suggesting the tax burden should be focused on corporations rather than individuals.

But Mr Khan said he would oppose a rise in corporation tax, pledging to be “the most business-friendly mayor of all time.”

Mr Corbyn finalised the details of his front bench yesterday, adding new leftwingers including Clive Lewis, Rebecca Long Bailey and Richard Burgon to his team.