The anti-Brexit party Change UK will this week unveil its list of 70 candidates for next month’s European elections after receiving more than 1,000 applications from former Labour, Tory, Liberal and Green party activists.

The party’s interim leader, the former Tory MP Heidi Allen, who yesterday challenged other leaders to a TV debate, claimed that the exodus from established parties showed Change UK was now the “natural home of the ‘Remain Alliance’” and of people who wanted a second referendum.

The party said that, of 3,700 people who had applied to stand under the Change UK banner in the 23 May poll, 895 were former Labour activists, 105 were ex-Liberal Democrats and 92 ex-Greens. Dozens of other applicants had been active in the Tory party, including former MPs. Of the Labour defectors, 32 were either former MPs, or had previously been parliamentary or council candidates.

The fledgling party, which will hold its launch on Tuesday in Bristol, knows it has to move fast after Nigel Farage’s Brexit party shot into an opinion poll lead over both Labour and the Tories just days after its launch.

According to the YouGov survey the Brexit party was on 23% when people were asked how they intended to vote in the European elections, with Labour on 22% and the Conservatives – many of whose members have said they will boycott them – on 17%. The Green party was on 10%, Change UK, which was registered as a political party only last week, was on 8%, and Farage’s former party, Ukip, on 6%.

Despite lagging behind, Allen said that the new party was building momentum: “From across the country and old party divides, people are coming together to tell Labour and Conservatives leaderships that they’ve had enough – enough of them focusing on self preservation and wilful facilitation of a damaging Brexit. There’s a real appetite building to support Change UK and send a clear message; give us a people’s vote with a chance to remain and get out of the Brexit mess.”

Change UK has faced criticism over recent days for refusing to unite with other pro-EU parties to prevent the Remain vote from splitting, and harming the chances of anti-Brexit parties winning seats under the proportional representation list system used for European elections. The Lib Dems, Greens, Change UK, the Scottish National party and Plaid Cymru all support the UK remaining in the EU and want to put the Brexit decision back to the people in another referendum.

The former Labour MP Chris Leslie, who defected to Change UK in February along with seven Labour colleagues and three Tory MPs, said the party was “happy to work across party lines arguing for a people’s vote” but was out to change the mould of British politics, which the Liberal Democrats had failed to do. “If the Lib Dems were the answer to Britain’s broken politics, we’d know that by now. Of course the existing parties won’t like new choice breaking through – but politics is changing and realigning. We need a fresh start and Change UK is the response to this.”

A Liberal Democrat spokesperson said the party had made overtures to Change UK to join forces but had been rebuffed.

“Knowing some anti-Brexit voters wanted a common front against the various Brexit forces – from Labour to Ukip and Farage – we were open to discussing what arrangements between parties could look like. Following conversations with people in Change UK and others, it became clear there wasn’t interest,” the spokesperson said.

The Liberal Democrats are expected to launch their European election campaign towards the end of this week. After agreeing to extend the UK’s EU membership until 31 October, leaders of the other 27 member states said the UK had to hold European elections unless a Brexit deal had passed through Parliament by 22 May. At the last European elections in 2014, Ukip won 24 seats, against 20 for Labour and 19 for the Conservatives.