Jess Phillips has announced that she is joining the race to replace Jeremy Corbyn as Labour leader.
The Birmingham Yardley MP joins shadow foreign secretary Emily Thornberry and shadow treasury minister Clive Lewis as confirmed candidates.
Other MPs considering a leadership bid include Sir Keir Starmer, Rebecca Long Bailey and Lisa Nandy.
Mr Corbyn is due to stand down in the wake of the party's election defeat last month.
Announcing her leadership bid in Grimsby, where Labour lost to the Conservatives at the election, Ms Phillips said that "something has to change" and that she was standing "because I think that we need more honesty in politics".
She acknowledged the campaign "won't necessarily be an easy fight" for her, but she thought Labour members were "ready to try something different".
Ms Phillips, a vocal critic of Mr Corbyn's leadership, said Labour needed a leader who would "truly speak truth to power" and be able to "take on" Prime Minister Boris Johnson.
"I'm able to reach people... and I'm able to get people to trust me even when they don't agree with me, and I think that's what politics needs."
In a statement, she said Labour would be in "big trouble" if it failed to win back support among its traditional base of working-class voters.
Ms Phillips also criticised the party's "woeful" response to anti-Semitism cases within the party.
And she warned the party against "trying to please everyone", which she said had usually meant "we have pleased no-one".
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.
At 38 years old, Ms Phillips is the youngest Labour MP to enter the leadership contest so far. 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.
Earlier, former deputy leader Tom Watson said 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.