Jess Phillips, an outspoken critic of current leader Mr Corbyn, launched her bid on Friday with a call to elect “a different kind of leader”.
Ms Phillips has been a long-term critic of his leadership. As a new MP in 2015, she told the Guardian: “I would do anything that I felt was going to make the Labour Party win the general election because if I don’t have that attitude then all I’m doing is colluding with the Tories.
“If that means making Jeremy better, I’ll roll my sleeves up. If that’s not going to happen, and I’ve said [this] to him and to his staff to their faces: ‘The day that... you are hurting us more than you are helping us, I won’t knife you in the back, I’ll knife you in the front.’”
She later said she regretted using this language. Earlier this year, Ms Phillips was one of Boris Johnson’s leading critics when he was accused of using inflammatory language in the Commons.
After Emily Thornberry and Clive Lewis, Ms Phillips has become the third MP to enter the race.
In a statement, she said: “We have got to be brave and bold and bring people with us, not try and look all ways. Trying to please everyone usually means we have pleased no one.
“Now is not the time to be meek. Boris Johnson needs to be challenged, with passion, heart and precision. We can beat him. We need to speak to people’s hearts, and people need to believe we really mean it when we do.
“Now is not the time to play it safe. What I’ve heard so far in this debate is totally inadequate to the scale of the problem. Voters have changed.
“We need to recognise that politics has changed in a fundamental way by electing a different kind of leader. More of the same will lead to more of the same result.
“We’re a party named after the working class who has lost huge parts of its working class base. Unless we address that, we are in big trouble.”
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MPs such as Rebecca Long-Bailey, Sir Keir Starmer and Lisa Nandy are expected to follow Ms Phillips and enter the race imminently.
Earlier this week, Ms Phillips came third in a YouGov survey of Labour members, behind Sir Keir and Ms Long-Bailey.