Change UK has called on Labour supporters to “lend us your vote” in order to put pressure on the Labour leadership to fully support a second Brexit referendum.

At an event in Westminster on Tuesday, which the party said was the first in a series around the country, the former Labour MP Chuka Umunna said that even if Labour did back a new referendum, it would only do so under a “whole list of terms and conditions”.

“My message to Londoners who voted Labour, and indeed to Labour members, is if you want to change the Labour party’s position on Brexit, don’t endorse that prevarication,” said Umunna, the party spokesman for Change UK, which is also known as the Independent Group.

“If you haven’t made up your mind what to do at the next general election, lend us your vote in these European elections. Because the better we do, the more likely you are to see the Labour leadership adopt a people’s vote and remain position.”

However, Mike Gapes – another one of the eight ex-Labour MPs who, along with three Conservatives, defected to set up the party – told the crowd of about 100 activists that Jeremy Corbyn was committed to supporting Brexit.

Gapes said Corbyn had voted against EU membership in the 1975 European referendum and had “kept the faith” ever since. “Corbyn whipped Labour MPs to vote for article 50, he whipped Labour MPs not to support a people’s vote,” he said.

The event heard from a series of the fledgling party’s MPs and MEP candidates, among them Gavin Esler, the former BBC journalist who tops their European elections list in the London region.

Gavin Esler tells the rally that Britain is ‘at a very serious, grave moment of decision’. Photograph: Nils Jorgensen/REX/Shutterstock

Esler contrasted the party’s view with that of Nigel Farage, whose Brexit party is currently topping opinion polling for the elections with up to 28%, well above Change UK on about 10%.

“We’re at a very serious, grave moment of decision, and there are two visions of this country,” Esler said.

“One comes from Nigel Farage, who claims we are racially divided, who wants to turn us away from real cooperation with Europe and who, like his friend, Donald Trump, has stirred up a meanness in our society. That is not my vision.

“If you want a poorer Britain, Nigel’s your man. If you want Brexit arguments for the next 10 years, the Tories and Labour I’m sure can fix that.”

The party’s campaign experienced a tricky start, with two would-be MEP candidates standing down within 24 hours of being unveiled, after offensive tweets they had sent emerged.

Another poll has shown that only 38% of voters have picked up the message that Change UK are an anti-Brexit party.

The party’s interim leader, former Conservative MP Heidi Allen, conceded there had been hiccups.

“I’m not so daft as to think it’ll be easy,” she said. “We’re learning an awful lot about setting up a brand new party very quickly. We haven’t got everything right and we won’t. That’s a fact. But what matters more is what we stand for.”