Rebecca Long-Bailey, the pro-Corbyn candidate for Labour leader, is finally entering the race, promising a "socialist agenda" and hitting out at her chief rival Sir Keir Starmer.
Announcing herself as a "proud socialist", the 40-year-old Shadow Business Secretary dismissed as the "continuity candidate" by opponents, becomes the sixth Labour MP to join the contest.
She joins Sir Keir, backbenchers Jess Phillips and Lisa Nandy, shadow Foreign Secretary Emily Thornberry and shadow Treasury minister Clive Lewis in an already-crowded field.
Writing on the left-wing Tribune website, she addresses party members as "comrades", accuses Labour of "cosying up to Rupert Murdoch" and should not have campaigned alongside David Cameron for Remain in the 2016 EU referendum.
And in what will be seen as a barbed attack on front runner Sir Keir - a top QC reputed to be a millionaire - and other rivals, she says she is "not a millionaire or a landlord, and I didn't go to a posh school".
Ms Long-Bailey declares: "Our task is to build a winning vision of a socialist future, and this task has never been more urgent.
"Many candidates in the leadership election say they will not return to the triangulation and Tory-lite policies that held our party back before Jeremy. 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. I don't just agree with the policies, I've spent the last four years writing them."
Reinforcing the view that she offers "continuity Corbyn", she writes: "Under the leadership of Jeremy Corbyn we've drawn upon the collective knowledge and experience of that movement to develop a radical, ambitious socialist vision for the future.
"This is our greatest strength, and we need a leader who comes from and will stay true to that movement."
And in what will be seen as a wooing of Labour left-wingers, she adds: "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."
In the 24 hours before her formal declaration, there had been claims that Ms Long-Bailey's campaign was faltering and that her camp was "panicking that they haven't got enough support". And her bid to win nominations from MPs was being made harder by the threat of the pro-Corbyn Labour Party chairman Ian Lavery to enter the contest, though he has now withdrawn.
But she says in her Tribune article: "I haven't rushed to announce my candidacy because I wanted to take time to reflect following the devastating results in December. I didn't emerge from the election with a ready-made leadership campaign because my every effort during the election went into campaigning for a Labour victory. I'm not driven by personal ambition, but by my principles and an unwavering desire to change our country and our world for the better.
"And those principles have led me here. I'm not your typical politician. I'm not a millionaire or a landlord, and I didn't go to a posh school. Instead I'm a lifelong socialist, dedicated to our movement and determined to do my bit. You're as likely to see me on a picket line as you are at the dispatch box, and you can trust me to fight the establishment tooth and nail."
And she concludes by quoting the hero of the socialist left and creator of the NHS in the 1940s, Aneurin Bevan, declaring: "We have a mountain to climb comrades, and the crises we face are stark. But we have our socialist vision, a path to victory and most importantly, we have each other.
"More than ever, Nye Bevan's words ring true: 'There is only one hope for humanity, and that is democratic socialism.'
"Our strength, determination and resilience will prevail. Together, we can do this."
To get on the ballot paper, Ms Long-Bailey and the other candidates need the nominations of at least 22 Labour MPs by next Monday and later at least 5% of all constituency parties and at least two trade unions.
Ms Long-Bailey received a boost when the left-wing Labour Party chairman Ian Lavery announced he had decided not to stand and would throw his full support behind her.
"Having worked with her in the shadow cabinet I know she has the intellect, drive and determination to take forward and develop the popular, common sense socialist policies that Jeremy Corbyn has championed," he said.
"And after more than a century it's about time the Labour Party was led by a woman."
The announcement by the Salford and Eccles MP that she is running comes just hours before the first hustings of the 12-week leadership campaign, before Labour MPs in the Commons.
The leadership hustings, to be chaired by the acting chair of the Parliamentary Labour Party, Catherine McKinnell, are at 6pm on the day MPs return to Westminster after the Christmas recess.
In an email to Labour MPs, leadership candidates have been given until 5pm to inform the PLP if they wish to address the hustings, which will last 90 minutes, until 7.30pm.
Further 90-minute hustings, for the deputy leadership of the party vacated by Tom Watson, will take place at 10am on Wednesday and would-be candidates have until 9am to ask to take part.
So far the declared candidates for deputy leader are Angela Rayner, who is backing Ms Long-Bailey for leader, Dawn Butler, Richard Burgon, Rosena Allin-Khan, Khalid Mahmood and Ian Murrary.
In the face for leader, in which the winner will be announced at a special conference on Grand National day, April 4, Sir Keir has opened up a solid lead over his rivals, according to the first poll of Labour members.
In the poll, by YouGov, 31% chose Sir Keir, 20% Ms Long-Bailey, 11% Ms Phillips, 7% Yvette Cooper and Clive Lewis, 6% Emily Thornberry and 5% Lisa Nandy.
The latest poll, by Sky Bet, quotes odds for Sir Keir of 4/6, with Ms Long-Bailey 3/1, Lisa Nandy 10/1, Jess Phillips 12/1, Clive Lewis 22/1 and Emily Thornberry 50/1.