Jess Phillips is expected to announce later that she is joining the race to replace Jeremy Corbyn as Labour leader.
The Birmingham Yardley MP would join shadow foreign secretary Emily Thornberry and shadow Treasury minister Clive Lewis in confirming her candidacy.
Other MPs considering a leadership bid include Sir Keir Starmer, Rebecca Long-Bailey and Lisa Nandy.
Mr Corbyn is due to stand down after the party’s election defeat last month.
Ms Phillips, who hinted on Twitter she would be entering the race, has been an outspoken critic of Mr Corbyn’s leadership and his handling of anti-Semitism cases within the party.
At 38 years old, she would be the youngest Labour MP to enter the leadership contest. She is also likely to be the only contender never to have held a position in the cabinet or shadow cabinet.
She has campaigned against Brexit, despite her Birmingham Yardley constituency, which she has represented since becoming an MP in 2015, opting for Leave in the 2016 referendum.
Shadow Brexit secretary Sir Keir Starmer and shadow business secretary Rebecca Long-Bailey have said they are considering running.
Wigan MP Lisa Nandy – who resigned from the shadow cabinet in 2016 after the Brexit referendum – is also considering throwing her hat into the ring.
There will also be an election for a new deputy leader, with shadow justice secretary Richard Burgon announcing his candidacy on Twitter, and shadow education secretary Angela Rayner receiving the backing of Ms Long-Bailey.
Under current rules, would-be candidates for both the leader and deputy leader roles must first be nominated by more than 20 MPs.
They must also secure nominations from at least 5% of Labour’s constituency parties or three affiliated bodies – two of which must be trade unions.
A timetable for the leadership election – and any rule changes – are set to be decided by the party’s National Executive Committee (NEC) on Monday.
Those contenders who – according to the admittedly limited polling we have – are more popular with the current left-wing membership would benefit from a more restricted timetable for the leadership contest.
Control of the NEC in recent years has moved to the left, so it’s unlikely the committee will want to be overly helpful to, say, arch-Corbyn critic Jess Phillips.
But a restricted timetable wouldn’t just potentially help Rebecca Long-Bailey, who has been dubbed by critics as a “continuity Corbyn candidate”.
It would likely also favour Sir Keir Starmer, whose pro-EU referendum stance and effective parliamentary performances seem to have, thus far, endeared him to a chunk of the largely pro-Remain membership.
Meanwhile, former deputy leader Tom Watson says the new leader’s “first task” will be to explain why the party has not won an election for a decade.
He added that shadow cabinet members wanting to succeed Mr Corbyn will face “particular pressure” over the party’s last manifesto.
Speaking on BBC Breakfast, Mr Watson said it was “too early in the race” to give his backing to any candidate.
He told the programme: “For whoever wants to lead the Labour Party, their first task is to explain to 500,000 members of the party why Labour lost and why Labour hasn’t won for a decade.”
Mr Watson said he “didn’t sign up” to the party’s 2019 manifesto, but he accepted he “must take responsibility for the failure as well”.
“So,” he added, “those members of the shadow cabinet that are running for leadership and deputy leadership of the party… have got a particular pressure on them, as they do have to explain whether they think that particular manifesto was the right one or not.”
Former Labour MP Melanie Onn – who lost her seat in Grimsby at the last election – said she would support Ms Phillips for the top job if she declared later.
She told the Today programme she was looking for a leader “who will transcend normal politics”, who would make “strong committed arguments”, and had a “strong personality” to stand up to Boris Johnson.
She said Ms Phillips, who visited her former constituency during the election campaign, was in a “very good position to reach out” to Labour voters who had moved away from the party.