Jeremy Corbyn has rejected calls from Labour MPs to pull out of a fundraising dinner for the Stop the War Coalition and said the group "has repeatedly called it right over 14 years of disastrous wars in the wider Middle East".
The Labour leader also insisted that the group, which Corbyn chaired until he was elected Labour leader, is a "vital democratic campaign".
Corbyn was forced to defend his attendance at the anti-war campaign's Christmas event after former shadow education secretary Tristram Hunt described Stop the War as a "disreputable" group and urged him on Sunday to step back from it.
In an appearance on BBC's Andrew Marr Show, Hunt also said the behaviour of Stop the War had been "irresponsible" before and after the Commons vote on Syria strikes last week.
But Corbyn has defiantly backed the anti-war group. A spokesperson for the Labour leader told The Guardian: "The anti-war movement has been a vital democratic campaign which organised the biggest demonstrations in British history and has repeatedly called it right over 14 years of disastrous wars in the wider Middle East.
"Jeremy Corbyn rejects any form of abuse in politics from any quarter. But he will not accept attempts to portray campaigning, lobbying and protest as somehow beyond the pale. In fact, it's at the heart of democracy."
Wes Streeting also piled on the pressure on Monday. He told BuzzFeed News: "I'm afraid there isn't a great deal of festive cheer towards the Stop the War Coalition, even amongst those, like me, who opposed air strikes in Syria."
Criticism of the group emerged as the anti-war group republished an article on its website in which journalist Matt Carr appeared to praise the "international solidarity" of ISIS.
Responding to the pro-airstrikes speech in Wednesday's debate by Hilary Benn, in which the shadow foreign secretary reminded Labour colleagues of the International Brigades of the Spanish Civil War, Carr wrote:
Stop the War praise the "internationalism and solidarity" of... ISIS https://t.co/4QGs9IXTGG
The blog post appears to have been taken down, but it is still available as a cached version online.
The Stop the War Coalition is using this image to sell tickets to the group's fundraising dinner, where seats are available at £50 a head.
Hunt, who this summer ran a brief campaign for the Labour leadership before withdrawing, said: "We've heard ugly comments [from the group] about Hilary Benn and the fact that Hilary Benn should be sacked.
"Their comments about Islamic State, about how the French almost had it coming to them. They're a really disreputable organisation and I wish that Jeremy Corbyn would step back."
He also criticised the group for organising a protest rally against the airstrikes at Labour headquarters in Westminster last Tuesday night that forced the party to abandon plans for a phone bank to support the party's candidate in the Oldham West by-election.
Speaking on BBC Radio 5 Live's Pienaar's Politics on Sunday, shadow energy and climate change secretary Lisa Nandy appeared to agree with Hunt. Responding to a question about the blog post, Nandy called it "extraordinary".
She continued: "I just think this debate has got way out of hand because of a small number of people who are aggressive on social media."
However, she suggested Corbyn could use his appearance to condemn recent tactics by the group.
"We need to send a very clear message from the top of the party and from the shadow cabinet that we don't tolerate deselection threats," Nandy said, "and I think if that means going to a Christmas dinner and making that point, I think that's fine. But i can't say whether he should or shouldn't go."