Jeremy Corbyn is "sorry" Frank Field resigned the Labour whip over the anti-Semitism row but does not see why the veteran MP had to step down.
The Labour leader insisted there had been a “democratic debate within the party” as he condemned bullying, saying it had "no place" in any political party.
Mr Field is facing calls to stand in a by-election after resigning the whip on Thursday after 40 years of representing Labour.
He is taking legal advice after party sources said it meant he had “automatically opted to resign from the Labour Party”, which he disputes.
The one-time minister for welfare reform plans to stand as an Independent.
He said that under Mr Corbyn the party was seeing an “erosion of our core values”, citing “a culture of intolerance, nastiness and intimidation” among local members.
Speaking in nearby Liverpool on Monday as he prepared to travel across the north of England campaigning on transport issues, Mr Corbyn said: “I’m sorry he’s resigned and I thank him for all the work that he’s done as an MP and for the party, but I don’t see why he had to resign."
"He's made his decision and obviously one has to acknowledge that," he added.
Asked about Mr Field’s claims of bullying in the party, the Labour leader said he did not know what the claims were, adding: “I invite all MPs to take part in the discussion and the debate.
“Obviously bullying and intimidation have no place whatsoever in any political party, particularly the Labour Party, but there has to be democratic debate within the party.”
Labour’s crisis over accusations of anti-Semitism is reaching a showdown moment as senior figures demand the leadership take firm action to prove the party is not hostile to the Jewish community.
Ahead of a crunch meeting of the ruling National Executive Committee on Tuesday, leading MPs have insisted the party must finally adopt the definition of anti-Semitism set out by the International Holocaust Remembrance Alliance (IHRA) in full.
The calls came after the issue continued to rock the party with former minister Dame Margaret Hodge standing by her claim that Jeremy Corbyn was an “anti-Semitic racist”.
Dame Margaret said that Mr Corbyn’s stance meant that the acceptance of the IHRA definition, and its examples, may no longer be enough.
Former prime minister Gordon Brown also insisted the IHRA definition must be adopted.