Jeremy Corbyn endured an “explosive” weekly gathering of MPs following the emergence of an anonymous briefing against the party’s deputy leader Tom Watson and a row over allegations of a left-wing “takeover plot”.

It came after the Labour leader and his deputy attempted to put on a united front by issuing a joint statement calling for unity after an “away day” of the party’s Shadow Cabinet at the Unison headquarters in central London on Monday.

In a joint statement the pair said: “The Shadow Cabinet met today to discuss Labour’s policy and election plans and had a robust and constructive discussion about the challenges and opportunities ahead”.

But shortly afterwards, briefings emerged that the deputy leader was “slapped down and left isolated” in the Shadow Cabinet meeting following a “reckless intervention” in Unite union’s internal elections. MPs at the PLP brought up the briefing, which Labour sources were furious over.

Over the weekend a leaked recording of Jon Lansman, the Momentum founder, emerged of him addressing supporters at a meeting in Richmond, South London, on 1 March. On the tape, obtained by The Observer, Mr Lansman said Unite would formally affiliate with Momentum, if Len McCluskey secured re-election as general secretary later this month.

Mr Watson then pleaded with Mr Corbyn to “deal with” Momentum and its founder in a series of radio and television interviews on Monday.

But despite the displays of unity from Labour’s leaders, outside the Parliamentary Labour Party (PLP) committee room, raised voices could be heard. When asked if the situation had any resemblance to the 1980s, one senior peer replied: “No, no – it is much, much worse than that”.

Another MP described the discussions over Mr Watson’s warning about the apparent plans of Momentum as “explosive”.

Reports also emerged of the party’s Shadow Foreign Secretary Emily Thornberry, urging one Labour MP, Wes Streeting, to “calm down”. One MP even confronted Seumas Milne, Labour’s director of communications, and told him he was a “disgrace”.

Speaking outside the PLP meeting room, a spokesman for the Labour leader described the conversation as “robust”. Denying he had briefed against the party’s deputy leader, he added: “I’ve been giving no such briefings.

“You'll know as well as we do that after PLP meetings in particular and after other Labour party meetings there's often briefing that goes on and the message that Jeremy gave to the meeting is that we need to be a united party and we need to be talking about the issues that affect our voters.

"That’s what we should be talking about, not the internal affairs of the Labour party. He'd encourage everybody who has been briefing about the internal affairs of the Labour party to follow that line.”

During the heated meeting Kezia Dugdale, the Scottish Labour leader, was also greeted with a rapturous applause after saying: “Never forget the best way we represent and deliver for working people will always be from the Government benches”. One MP described her speech as “brilliant” outside the committee room.

Earlier on Monday Mr Watson was publicly condemned by Mr Corbyn’s biggest union backer over the comments he made regarding Momentum’s apparent plan to affiliate to the union. Dismissing the affiliation claims as “entirely inaccurate. The union, however, accused Mr Watson of a “misguided” campaign to interfere in Unite’s internal democracy and its ongoing general secretary election.

Jon Lansman, the founder of Momentum, attempted to calm tensions on Monday. He told The Independent: “There are no secret deals or plots to take over Labour.

“As Tom Watson acknowledged in his joint unity statement with Jeremy Corbyn, groups across the spectrum of Labour’s broad church have the right to discuss their views and try to influence the party.