Confirming her leadership bid, Ms Long-Bailey, the leading candidate from the left, said she was a "proud socialist" that would protect Jeremy Corbyn's legacy, and criticised the winning pro-Union campaign as an example of Labour being "too close to the establishment".

With nominations due to open on Tuesday, Ms Long-Bailey - favourite of the Labour left - said she could be trusted to maintain "our socialist agenda".

Her comments will be seen as a thinly veiled swipe at shadow Brexit secretary Sir Keir Starmer, who has been keen to stress his left-wing credentials despite being seen to come from a more centrist tradition.

Shadow Business Secretary Rebecca Long-Bailey. Picture: Getty

Rumoured runners and riders to replace Jeremy Corbyn as Labour leader

"But we need a leader that can be trusted with our socialist agenda. A leader who is totally committed to the policies and has the political backbone to defend them.

"We need a proud socialist to lead the Labour Party, driven by their principles and an unwavering determination to see democratic socialism in our lifetime.

"For all of these reasons and more, I have decided to stand for election to become the next leader of our party."

She added: "We've also, at times, been too close to the establishment we are meant to be taking on - whether cosying up to Rupert Murdoch, joining forces with David Cameron in the Better Together campaign in 2014 or turning our focus inwards on parliamentary manoeuvring for the last year.

"To win, we need to rebuild Labour as an insurgent force and offer a vision for a new democracy. We must go to war with the political establishment, pledging a constitutional revolution that sweeps away the House of Lords, takes big money out of politics and radically shifts power away from Westminster."

Party chairman Ian Lavery immediately announced he would not be standing and would be backing Ms Long-Bailey.

"We must ensure that we never again are seen to be taking working class communities for granted or to write them off as ignorant or ill-educated," he said in another apparent jibe at pro-Remain contenders.

Ms Long-Bailey's announcement came as the party's ruling National Executive Committee (NEC) set out the timetable for election, with the new leader to be announced at a conference on Saturday April 4.

The NEC also confirmed that, as in 2016, "registered supporters" will be able to vote if they pay a £25 fee.

A party spokeswoman said there would be a 48-hour window to apply from 5pm Tuesday January 14.

"We are by far the largest political party in the UK with well over half a million members," the spokeswoman said.

"We want as many of our members and supporters to take part, so it has been designed to be open, fair and democratic."

Under the terms of the contest, candidates need to secure the nominations of least 10% (22) of the party's MPs and MEPs.

Those who succeed will go forward to the second stage where they must get the nominations of 5% of constituency Labour parties or three Labour affiliates - of which at least two must be trade unions - comprising at least 5% of the fully paid-up affiliate membership.

The closing date for new members to join and be eligible to vote will be January 20 in the postal ballot.

Rebecca Long-Bailey attacks Labour invovlement in Better Together