Jeremy Corbyn has strongly defended Momentum, the group set up to champion his leftwing agenda which has been denounced by members of the shadow cabinet.

In an interview with the Morning Star, the Labour leader said Momentum could attract a lot of new people into the party and that this would be “a good thing”. Labour could not expand if it just focused on existing members, he said.

Momentum was set up after Corbyn’s election as party leader, as the successor organisation to his leadership campaign. It is open to people who are not Labour members, and some MPs who do not support Corbyn are alarmed by its rise because they fear it will enable people previously involved in far-left groups outside Labour to exercise influence in the party.

Corbyn said he was proud his leadership campaign had managed to mobilise supporters who in the past were not involved in politics and said he envisaged Momentum carrying on this work. “It could be a way of bringing in a lot of people,” he said. “That’s got to be a good thing. If the Labour party is going to expand it’s got to bring in people who weren’t in it before. You cannot recruit only from your own membership.”

Corbyn said Labour party membership was now almost 400,000, which was “the biggest, most active membership ever in my lifetime”. But he also said Momentum “should focus on the Labour party and on Labour activity”.

Some of Corbyn’s shadow cabinet colleagues have taken a different stance on Momentum. Earlier this month Tom Watson, the deputy leader, said Momentum “look like a bit of a rabble” and that they were an “irrelevance”.

Michael Dugher, the shadow culture secretary, said he could not understand why Corbyn supporters needed their own organisation when they had the Labour party, with Corbyn as leader. He said there was “no need to create a new faction in the Labour party, which has been susceptible to entryists and which has at times resembled the mob”.

Momentum is still establishing itself as an organisation and recently said it would be drawing up rules to ensure people who are members of other parties cannot participate in its decision-making meetings.

In his Morning Star interview, Corbyn also seemed to rule out Labour striking some sort of electoral deal with the Green party. He said he had huge respect for Caroline Lucas, the Green MP, and that he had campaigned with her on many issues. “But I’m not proposing electoral pacts or coalitions,” he said. “Lots of people who hitherto voted Green are coming round to supporting Labour now.”

Corbyn also claimed that the government was trying to eliminate the labour movement. The trade union bill, which will make it harder for unions to call strikes and change political contribution rules, the cut to Short money for opposition parties, the move to individual voter registration and the restrictions on charity campaigning during elections, were all part of this, he said.

“What they’re doing is actually questioning the existence of labour movement politics altogether,” he said. “We’ve got to stand up for what we believe in as a labour movement. And that means the party’s membership needs to be even bigger, so it becomes a genuinely mass organisation.”