People expelled from Labour after exhaustive disciplinary inquiries will still be welcome inside the leftwing grassroots organisation Momentum – despite a new constitution designed to pave the way for its formal affiliation with the party.

Last week, the Observer revealed details of a secret recording of a meeting at which Momentum’s founder, Jon Lansman, outlined plans for its activists to build influence at all levels of the party, in order to ensure a hard-left successor is installed when Jeremy Corbyn departs. Lansman also said Momentum would affiliate with the union Unite, boosting its finances, if Len McCluskey remained as general secretary, although a spokesperson later said he had been speaking in an “aspirational” manner and Unite insisted there were no such plans.

Lansman made clear on tape that his aim was to require Labour membership in the rule book in order that Momentum could build formal links with the party and the country’s biggest and richest union. The revelations prompted Labour’s deputy leader, Tom Watson, to warn that the future of the party was at stake from a wave of “entryism” as serious as any since the battles with Militant in the 1980s.

It can now also be revealed that Lansman reassured activists at the same taped meeting that people expelled from Labour would still be welcome to participate in its activities, if not in its ruling bodies and committees. This is despite Momentum’s new constitution saying that any new member has to be a Labour member and that all existing ones should join. Asked by an activist at the meeting in Richmond, south-west London, on 1 March about what role people suspended or expelled from Labour could play in Momentum in future, Lansman stated: “I see no reason why they should not carry on. On the basis that most people who have been expelled have been expelled unfairly, – the [Labour party] compliance unit has been trying to kick them out – there is no one in Momentum who wishes to exclude people.”

The comments suggest that Lansman is nervous about enforcing the constitution, while at the same time claiming his organisation only contains Labour members, making it eligible to affiliate with Labour and Unite.

Speaking at Momentum’s first conference yesterday in Birmingham, the shadow chancellor John McDonnell, a close ally of Corbyn, defended the rights of Momentum activists to get involved in party structures, but insisted this be done in a “comradely” manner. “It means offering to serve. If there’s a vacancy in your constituency party, volunteer for it. If there’s an exercise that’s being done, if it’s leafleting or canvassing, offer to do it. In that way you win the respect of others.”

Responding to accusations made on Twitter last week by the Labour MP Jess Phillips that he had “lied” to Unite members, Lansman said that non-Labour members would be welcome to participate, but excluded from standing or voting in internal elections. Lansman wrote: “We won’t allow non Lab members to hold office or vote (unlike Coop party or Fabians) but we won’t exclude them from activities/meetings”.

Momentum’s new constitution states that all new members have to be Labour party members and that “existing members who are not already Labour Party members are being encouraged to join Labour by 1 July.” It adds that “Momentum members who believe they have been expelled from the Labour party unfairly have the right to a hearing by Momentum’s National Coordinating Group.”

A Momentum spokesperson said it was a “non story” to point out Mr Lansman’s comments about expelled Labour members being welcome to take part.She said: “As both our constitution and and website clearly state, any existing members of Momentum who are not Labour members can participate in local campaigns and events but cannot vote in election, stand for an official position or hold other membership rights.”

■ Labour sources said a decision on whether to expel the former London mayor Ken Livingstone from Labour over alleged antisemitic remarks last year will be announced by the party this week. Livingstone has said that if the decision is to expel him, he will take action in the courts.