Veteran Labour MP Dame Margaret Hodge has said she is "disappointed" to be facing a reselection contest in her east London constituency.
Dame Margaret, who is Jewish, has been critical of Jeremy Corbyn's leadership and the party's response to complaints of anti-Semitism.
It comes after activists in her constituency voted for the chance to pick a new candidate.
The MP for Barking said she would fight to keep her seat.
She is the second Labour MP to face the party's new ballot procedures which make it easier for local members to force a contest.
Jon Lansman - chair of the pro-Corbyn grassroots movement Momentum - tweeted "the Left is most certainly not behind the triggering" of the reselection contest.
In a statement, Dame Margaret said she was "obviously disappointed" but her priority remained "serving the people of Barking as I have done for the last 25 years".
"At a vital time for the country, with a general election looming, we should be focusing our efforts on holding Boris Johnson and the Tories to account," she said.
"I will work to secure the full backing of Barking Labour Party, so I can continue to play my part as their MP in doing that."
Dame Margaret has been an outspoken critic of how the party has dealt with claims of anti-Semitism.
Mike Katz, chairman of the Jewish Labour Movement, said the trigger ballot vote was a "shameful moment for a party which claims to embody the values of equality and diversity".
"Margaret Hodge has been a steadfast campaigner against racism, fascism and intolerance throughout her political life," he said.
"She saw off the BNP in Barking and has, over the last few years, been determined in her opposition to anti-Semitism within the Labour Party."
"She has been the target of vicious smears, derision and anti-Semitism, because she has been resolute in her opposition to anti-Jewish racism," he added.
Labour's former deputy leader Harriet Harman said she was "dismayed more than I can say" by the vote.
She tweeted: "Margaret Hodge, scourge of tax avoiders, racists and anti-Semitism. Champion of progressive policies. Hyperactive local MP. Surely this cannot stand!"
In his tweet, Mr Lansman insisted the triggered contest was not a consequence of anti-Semitism.
"I'm told it happened because members on the Right had expected her to retire and want an MP who actually lives there, not one who has always lived in Islington," he wrote.
A Labour Party spokesperson said: "Under longstanding party rules, reselection processes have to be held in between general elections."
"These are taking place in every Labour-held constituency across the country and in any reselection sitting MPs are automatically on the ballot paper," the spokesperson added.
A party source echoed Mr Lansman, saying there was nothing to suggest the vote had anything to do with anti-Semitism.
"Many members wish to hold democratic selections so they can have a say in who represents them," the source said.