Chuka Umunna has said he wants to “play the biggest role” in the breakaway Independent Group (TIG) of MPs, whose 11 members will hold their first formal meeting on Monday.

Umunna, who represents Streatham in south London, is one of eight former Labour MPs who have joined forces with three former Tories to make a new group in the House of Commons.

TIG does not have a leader or a detailed policy platform, but an Opinium poll for the Observer puts the group’s support at 6%, a point ahead of the Lib Dems.

It has emerged that Sir David Garrard, once one of Labour’s biggest donors, is supporting the new group.

Asked by Sophy Ridge on Sky News whether he was the group’s leader, Umunna said, “Well I’m not,” but quickly added: “I’m clear I want to play the biggest role in this group. And I think one of the things about the way we operate is the recognition that we’re all leaders.”

Barry Gardiner, the shadow international trade secretary, claimed on Sunday that Umunna had only left Labour because he knew he would never become its leader.

“Here was somebody who had no great policy difference; it was all about personality,” Gardiner told the BBC’s Pienaar’s Politics. “It was fairly clear to me that in effect the reason that he wanted to leave the Labour party was because he knew he could never be the leader of the Labour party and this is about personality.”

Umunna and his 10 colleagues have not yet formed a political party but they have suggested they are keen to move towards doing so.

Chris Leslie told the Guardian on Friday: “We’ve said we’re not a party yet, but I think it’s something that we’ve got to test out, and hear what the public appetite is.”

TIG MPs have signed up to a statement of principles but there have been questions about how they will assemble a coherent set of policies to which they can all sign up, given their widely differing past voting records on issues including austerity.

Former Conservative Anna Soubry told journalists at a press conference last week she believed Tory chancellor George Osborne had done “a marvellous job” during the 2010-15 coalition government.

But ex-Labour MP Luciana Berger said on Sunday: “We’re not going to agree on everything; we’re coming from different political traditions, we’re coming from different generations,” adding: “We’re going to operate in an adult way, which is beyond party lines.”

TIG’s MPs all campaigned for Britain to remain in the EU, and Umunna has been a prominent supporter of a second Brexit referendum.

Umunna, a former shadow business secretary and Labour leadership contender, has been described as the obvious candidate to lead TIG in the Commons, where the 11-strong group equals the Lib Dems in size and outnumbers the DUP by one.

There has been continued speculation that more Labour and Tory MPs will quit and potentially join the new group in the coming days and weeks as Brexit comes to a head. The departures already mark the biggest parliamentary schism since the formation of the Social Democratic party in the 1980s.

Many of TIG’s MPs will face a huge challenge, however, if they want to hold on to their seats in a future election.

The Labour leader, Jeremy Corbyn, and several members of his shadow cabinet staged a rally in Soubry’s Broxtowe constituency on Saturday.

The shadow foreign secretary, Emily Thornberry, said MPs who left Labour had betrayed their seats and would be defeated if elections were held.

Fighting for seats in a general election will require significant funding, although TIG has already attracted support from major donors who have abandoned the main parties in recent months.

Umunna has claimed that the group has already raised “tens of thousands of pounds from several thousand small donors”.

Garrard told the Observer on Sunday: “I’ve been asked whether I will financially support the new group. I have already done so.”

The property tycoon, who donated around £1.5m to Labour over a period of 15 years, left the party in protest at the leadership’s handling of the antisemitism row.