Labour's hard-left rivals are attempting to infiltrate the party through the pro-Jeremy Corbyn pressure group, according to Tom Watson. The deputy leader claimed some of the organisation's branches had an "entryist problem" and Trotskyist groups such as the Socialist Party of England and Wales (formerly known as the Militant Tendency) were trying to gain Labour membership through Momentum.

Watson made the claim after he was challenged on Twitter by Labour activist Éoin Clarke after describing Momentum, which is an offshoot of Corbyn's leadership campaign, as a rabble. Clarke took exception to the deputy leader's comments and claimed many in Momentum had campaigned to get Watson in power. "If you choose to take it personally, I'm sorry. It doesn't change the fact they have an entryist problem in a number of groups," the West Bromwich East MP replied.

The comments come after Nancy Taaffe, a Socialist Party member and Trade Unionist and Socialist Coalition (TUSC) candidate at the election, called for the deselection of Labour Walthamstow MP Stella Creasy. Jon Lansman, a director of Momentum, described Taffe's attack as hostile and opportunistic.

The Socialist Workers Party (SWP), a separate hard-left group, has openly called for its supporters to cooperate with Momentum. "The new group's stated aim is to pull people into Labour – at the moment it is not a membership organisation and is calling for supporters," a statement on the party's website said. "This is a chance for activists in and out of Labour to cooperate in activity. It increases the space for debate about how to fight back.

"Momentum's call to mobilise those people to be active in their localities with others on the left can help that political mood. And it can feed into strengthening working class organisation. Socialists in and outside Labour should support it."

But James Schneider, a spokesman for Momentum, has previously denied entryist claims. "No, we're not the party within, and no we will not be campaigning to deselect members of parliament in any capacity. Momentum is not designed to engage in that kind of activity," he told BBC 2's Daily Politics show.

Schneider declined to comment on 8 December. Meanwhile, a Socialist Party source told IBTimes UK that Momentum was "sucking up" to the right-wing of Labour.