Labour MP Jess Phillips is urging people to join the Labour Party in a bid to help choose the new leader to replace Jeremy Corbyn.
The Birmingham Yardley politician, who is an outsider to run herself, told her 333,000 followers on Twitter that they likely have until January 20 to join the party to affect change.
The Labour Party Rule book states that people who join within two weeks of the announcement of the leadership timetable will be allowed to vote.
The date of January 7th has been mooted for the party's National Executive Committee (NEC) to decide the timetable, meaning the two week deadline would be January 20.
Ms Phillips said: "Don't worry about which sort of Labour you arem if you want rid of Johnson's government and want a say in how, join now."
The rules for membership of the party were changed under Ed Miliband, with around half a million people now thought to be part of Labour.
The call to join comes after a bad tempered meeting of Labour's remaining MPs in Parliament last night.
Jeremy Corbyn was subjected to extensive criticism after he lead the party to it's worst defeat since 1935.
Some MPs and defeated candidates have been levelling the blame squarely at Mr Corbyn, saying his past record and the volume of his left-wing policies were damaging on the doorstep.
Mr Corbyn apologised to a fractious meeting of the Parliamentary Labour Party (PLP), which was attended by several potential successors and many vocal critics.
“I am very sorry for the result for which I take responsibility,” he told them.
Mr Corbyn maintained his defence that Brexit was a major reason voters lost their trust in Labour and repeated his criticism of the media.
Ms Phillips said the meeting was largely critical of Mr Corbyn with only “a couple of people being supportive”.
Veteran Labour MP Dame Margaret Hodge, who has long been critical of Mr Corbyn, said that “on the whole it was fury, despair, miserable and I just felt that the top table had corporate amnesia”.
Ilford North MP Wes Streeting criticised Mr Corbyn for carrying on “just like business as usual”, adding: “Nothing about the tone and content of what we heard reflected an election defeat worse than 1935.”
Some Labour MPs said, however, that Mr Corbyn was not told to go immediately with them apparently supporting his plan to depart after overseeing a “process of reflection”.
The new leader is expected to be in place by the end of March.
