On Friday evening at 7.55pm, defiant despite a barrage of criticism, Jeremy Corbyn arrived at a Turkish restaurant in Southwark, south London to attend the Stop the War Coalition’s Christmas bash. Entering discreetly via a back door, the Labour leader was wearing his distinctive black Breton cap. Two-and-a-half hours later, he emerged bare-headed, having offered the headwear for auction. The winning bidder paid £270 for the privilege of taking it away – perhaps to wear during the next day’s Stop the War march on Downing Street.

Well, if the cap fits.

This was a pretty clear message about the Labour leader’s allegiances – although he then went one step further. In a 15-minute speech, in which he acknowledged Stop the War’s faults, and joked about the abuse he had received in recent weeks, Corbyn insisted to cheers from the 200 people eating meze: “The antiwar movement has been a vital force at the heart of our democracy. Branding it as somehow illegitimate is an attempt to close down democratic debate and campaigning.”

On paper this has been one of the more bizarre political rows. Corbyn, outgoing chair of the Stop the War Coalition – formed in the week following the 9/11 terror attacks in reaction to the plans of President George W Bush – had been intent on breaking flatbread with his friends at the group’s Christmas fundraiser. Those on the right of his party said that in doing so he would be showing “two fingers” to Labour. A now-disowned statement on the Stop of the War website, in which it was claimed that last month’s Paris terror attacks were an example of the west reaping its reward for waging war in the Middle East, was one reason why attending the party was said to be so offensive. The former shadow education secretary, Tristram Hunt, had described Stop the War as “disreputable”; others said it had “lost its moral compass”.

But if the critics believed that Corbyn had any intention of pulling out, they were clearly wrong. And John Rees, a founding member of the coalition and now its national officer, thinks he knows why. “I think we understand what is going on here,” said Rees, a former member of the Socialist Workers Party who describes himself as a Marxist. “The Labour right understand that a source of Jeremy’s popularity, and root of his success, are his links with a broad social movement and they are trying to make a wedge between those two things.

“They are in any case, pro-intervention and pro-war themselves, so for them they do not like the substantive issue of opposition to war and they don’t like the fact that their opponent inside the Labour party has that kind of popular support.

“They would like to separate Jeremy from the movement that gave him popular support. And make it easier for him to be replaced at a later date. That is in essence the Peter Mandelson plan – give him time to disappoint his supporters.”

In reference to Hunt’s comments, Rees added that he believed the relationship between Corbyn and the group would only strengthen: “What is disreputable is to use an issue like war and peace to further your argument inside the Labour party in a factional basis. That is not a very pleasant game really.

“We have known Jeremy for a long time. We will talk to him, as will hundreds of other organisations. There isn’t going to be a mechanical, instrumental relationship, but common politics voluntarily entered into, which shapes how they [Labour] respond. I suspect that as challenges come up I expect Jeremy’s reaction will be very similar to our reactions.”

Cap in hand: Corbyn sold off his headgear in a charity auction for £270. Photograph: Dan Kitwood/Getty Images

John McDonnell, the shadow chancellor, who only missed the party in order to attend and oversee the raffle at his own constituency Christmas party, added: “In terms of Jeremy and myself we are just behaving as normal. Just carrying on as we normally do. And one of the things we normally do is campaign against unjust wars.

“That is why we were involved in the foundation of Stop the War. Again, others have been critical of Stop the War and some of the positions they have taken, but that is honest political debate.

“As far as I am concerned, Stop the War have got it right in terms of Iraq and Afghanistan and in terms of the bombing of Syria. So of course we continue to support the organisation.”

None of this will give Corbyn’s critics much festive cheer. But if McDonnell’s comments about Stop the War dampen the spirits of some parts of the Labour party, his clear intentions with regard to an activist group accused of bullying MPs into opposing bombing in Syria will further dismay them.

There have been calls to disband Momentum, which grew out of Corbyn’s leadership campaign. The group has been described by deputy leader Tom Watson as a rabble and some Labour MPs fear it is a vehicle for the ultra-left. But McDonnell wants to deepen ties with its 70,000 activists across the country. They were similar to the Fabian Society, he said. “Momentum members are being encouraged to join the Labour party, and most of them are now,” McDonnell said.

“The NEC [Labour’s ruling body] are looking at innovative ways of involving all Labour members in democratic policymaking through social media and that will come in the new year. Meanwhile Momentum will publish policy papers, like the Fabians and countless others.”

McDonnell, who said the work of 200 Momentum canvassers in the recent Oldham byelection had been instrumental in bringing Labour their healthy majority, added: “The whole point of Momentum was that there were lots and lots of young people, especially, who had campaigned throughout Jeremy’s campaign.

“Tens of thousands of young people came into his campaign and at the end they wanted to keep on campaigning and we welcomed that. It was just great to have that number of young people engaged in politics. They are relatively naive, many have not been involved in party politics and we have welcomed it. They model themselves on the Fabian society. They are learning lessons as they go on.”

Leaving the Turkish restaurant on Friday night, one of those who paid £50 for the evening, Steve Williams, a Labour party member for 45 years, spoke for the room. “It was an excellent evening, it was uplifting, and it was reinvigorating for those of us who have been working for many, many years in the Labour party. I’ve come out [tonight] with a feeling that we have a leader of the Labour party who is actually a principled man, a man of integrity, a man of peace, a man who is actually going to take this country forward as prime minister.”

Divide and rule may or may not have been the strategy of some Labour right-wingers. But Corbyn and his allies aren’t going to be separated any time soon.