Change UK has launched its campaign for the European Parliamentary elections, with 70 candidates including journalist Rachel Johnson - sister of Tory MP and leading Brexiteer Boris Johnson.
The party - formerly known as The Independent Group - is made up of 11 MPs who quit Labour and the Tories.
They are preparing for the European election as the latest Brexit delay means the UK may have to take part.
Change UK are a pro-Remain party who back another referendum on Brexit.
Ms Johnson, who was most recently a member of the Liberal Democrats, said: "I'm sure that Boris understands why this is not a vote against Boris.
"This is a vote for change. We need to move the dial. People need to have a say."
Interim leader Heidi Allen told the event in Bristol: "These elections are a chance to send the clearest possible message - we demand a People's Vote and the right to campaign to remain in the European Union. We are not afraid to say it as clearly as that.
"This is no rebel alliance. This is the home of the Remain alliance."
Former BBC correspondent Gavin Esler is also a candidate. He said UK politics was "a worldwide joke", and accused pro-Brexit campaigners, such as Nigel Farage, of "stealing our patriotism".
"We know Britain is better than the brain-dead politics of the past. We know Change UK is the only all-UK party to bring about changes.
"Ask yourself, what time is it? It is time for a change - it is time for Change UK."
BBC political correspondent Nick Eardley says Change UK want to be the number one choice for those unhappy with Brexit.
But he said it would not be easy because plenty of other more established parties - namely the Lib Dems, the Greens, the SNP and Plaid Cymru - will be making a similar pitch.
Following the launch, the Independent reported that a potential Change UK candidate, Ali Sadjady, had tweeted a derogatory comment about Romanians in November 2017. The tweet has now been deleted.
The party later said that "following discussions", Mr Sadjady, had "reflected on his inappropriate tweet from 2017 and agreed to stand down".
Mr Sadjady posted an apology on Twitter, saying it was not representative of who he was.
The beginnings of Change UK
A number of parties have begun launching their campaigns for the European Parliamentary elections after Theresa May agreed a Brexit delay until 31 October with the EU.
Mrs May said the UK could still exit before this date if her withdrawal agreement is approved by Parliament.
But, if not, the country is likely to have to hold European Parliament elections on 23 May.
The Independent Group was formed after eight Labour MPs and three Tory MPs left their parties in February.
The three Conservative MPs blamed "a shift to the right" in their party and the government's "disastrous handling of Brexit", while Labour MP cited their party's stance on leaving the EU, along with the leadership's handling of anti-Semitism.
In March, the group applied to become a party with the name Change UK and was given approval in April, meaning they could field candidates in the European elections.
After about 3,700 applications, 70 candidates were picked.
Change UK MP Anna Soubry said: "No ifs no buts, no backroom deals... we were never on the fence, we always believed it and now we demand it... a People's Vote.
"If MPs can change their minds and change their votes, so can the British people. It is undemocratic, it is plain wrong, to deny people the rights of this country that are enjoyed in Parliament."
Who is Rachel Johnson?
The sister of former Foreign Secretary Boris Johnson is a writer and newspaper columnist.
She has previously said she joined the Conservatives in 2008, inspired by a dinner party discussion with David Cameron, but left in 2011, complaining she was treated "like the brainwashed member of a cult".
In April 2017, she joined the Liberal Democrats, saying that the country could "be going over a cliff in a hard Brexit".
Ms Johnson was a contestant on Celebrity Big Brother in 2018.
Who is Gavin Esler?
Glasgow-born Gavin Esler started a career in journalism at the The Belfast Telegraph before joining the BBC during the Northern Ireland Troubles.
During a long career at the corporation, he was the BBC's chief North America correspondent, based in Washington, and a presenter of Newsnight. He left in 2017.
He has written five novels and two non-fiction books, and is currently Chancellor of the University of Kent.
He spoke in favour of a "final say on the Brexit deal" at a rally for the People's Vote campaign in Scotland in August last year.
Brexit Party announces new candidates
Last week, ex-UKIP leader Mr Farage launched his new Brexit Party, and on Tuesday, the party announced five new candidates for the European elections.
They include left-wing activist Claire Fox, former Royal Marine James Glancy and charity CEO Matthew Patten.
Nigel Farage said the group would be targeting areas with Labour MPs that voted Leave in the referendum.
He added: "Brexit is not about left or right. It is about right and wrong."
Who is standing in the European elections?
The European Parliamentary elections are expected to take place in the UK on 23 May - if the UK has not left the EU by that date.
The UK is divided into 12 regions, each represented by between three and 10 MEPs depending on population size.
Seats in England, Scotland and Wales are awarded to parties according to their share of the vote, to candidates on lists drawn up by the parties.
Northern Ireland elects MEPs using a single transferable vote system, with voters able to rank candidates in order of preference.
The deadline for nominations for candidates is 16:00 BST on Wednesday 24 April for the South West Region (including Gibraltar) and 16:00 on Thursday 25 for all other candidates.
But the lists of candidates announced so far include: