Labour should consider banning Momentum members from the party, Owen Smith signaled today.
Asked whether he would support banning the organisation, which was set up to support Labour leader Jeremy Corbyn, Smith suggested the NEC should "look carefully" at it.
"If Momentum carries on being a party within a party, then that is something everybody needs to look carefully at," he told reporters at an event in London.
He said that while it was not for him to tell Labour's ruling committee what should be done about the organisation, it was "not helpful" for it to continue to operate.
"I'm not going to get to the point of telling the NEC what they should be doing about this at all. That's for them to consider that. But I don't think it's helpful to have parties within parties."
Smith described Momentum as an alien creature which was parasitically using the Labour party as a host organism.
"There is nothing comradely about setting up party within a party. Still less in trying to use our movement as a host body, seeking to occupy it, hollow it out, until it’s outlived its usefulness, when you throw it aside like a dead husk."
Smith suggested that Momentum was in reality a re-branding of the Militant tendency in the 1980s.
"We have seen these tactics in the past under Militant. Momentum is not terribly subtle. Creating a big 'M' at the front of their name should give the game away."
He said that Momentum had to be fought in the same way now that Militant were then.
"We had to fight Militant in the 1980s, we have to fight to save the party we love today…
"Momentum in Brighton and Liverpool - some of them exactly the same people as were in Militant all those years ago - organising to deselect a Labour MP."
Militant were a Trotskyist group engaged in 'entryism' into the Labour party in the 1980s. They were eventually made a proscribed organisation, with members expelled from the party.
One source at Momentum accused Smith and Corbyn's other opponents in the party of hypocrisy for their attacks on the organisation.
"Selections are a matter for local party members and affiliates, who have the right to choose their officers and representatives through agreed democratic procedures," they told Politics.co.uk.
"It is ironic that some of those who tried to deselect Jeremy as leader, without a vote and without following agreed procedures, have decided that Labour members and affliates having the right to select their candidates in accordance with party rules is somehow abusive."
Smith's comments about Momentum mark a departure from his previous attempts to woo the organisation's members. At the end of July, the Chair of Smith's camapign, Kate Green, wrote to Momentum chief Jon Lansman asking him to arrange a leadership debate hosted by the organisation.
"I write to propose that Owen join Jeremy to debate their visions for the future of our movement at a Momentum-organised meeting (or series of meetings), in Islington, or anywhere in the country, as soon as possible," she wrote.
"Your assistance in arranging this would be appreciated, and I look forward to hearing from you."
Responding to Smith's comments, a spokesperson for Momentum defended the organisation.
"Momentum brings together the enthusiasm and excitement in the Labour Party caused by Jeremy's leadership. We seek to strengthen the party and help it win elections by making it a more participatory, democratic and campaigning party. We will work with people and organisations right across the labour movement to achieve these ends and transform society in the interests of the many."