The Independent Group of MPs who defected from Labour and the Conservatives have applied to become a political party and stand in the EU elections in May.
It comes as the House of Commons prepares to vote on Theresa May’s EU withdrawal agreement, with the risk that Britain will be forced to partake in the European elections greatly increasing if it does not pass.
Ms Allen said: “Today marks a huge step forward on The Independent Group’s journey to becoming a fully fledged political party, so I am delighted to have been chosen as our interim leader.
“If we are to deliver on our ambition to change politics for the better, it is vital that we attract support from people from every walk of life, every political background and none.”
The group was born when MPs who left Jeremy Corbyn’s Labour party over antisemitism and his Brexit stance sat together in the House of Commons and were then later joined by three Tory MPs who left their party over Ms May’s approach to EU withdrawal.
Change UK’s new group spokesman Chuka Umunna claimed the new party had been “overwhelmed by the public support”.
He said: “There is clearly an appetite for an alternative to our broken politics which needs fundamental change, as shown by the disastrous Brexit process that has occurred under the watch of the two main parties.
“A new party will shake up the two-party system and provide people with an alternative that can change our country for the better.”
The party will stand candidates as MEPs across the UK if their application is approved by the Electoral Commission in the next few weeks.
However, they are not planning to put up candidates in local elections in England in May, as the timetable is too tight, party sources said.
An MP involved said: “The ground is moving underneath our feet very quickly indeed so we have to get ourselves fit.
“We had wanted to proceed a little more slowly, building our policy and our manifesto – and we will continue with that work – but we need to crack on.
“Not just because these elections are looming but also because of the groundswell of support across the country – people are just so eager for us to do this.”
Individual donations have poured in from the public to fund the group, although it does not have any large financial backers, party sources said.
Another source dismissed claims they had not yet registered as a party to avoid financial scrutiny as “absolute bollocks”.
South Cambridgeshire MP Ms Allen was unanimously chosen as interim leader as part of efforts to attract people from outside of politics, choosing “somebody who doesn’t have the normal background of a politico or a special adviser”.
A source compared the relationship between Ms Allen and Mr Umunna to a firm’s chairman and chief executive, saying they would make “a really great double act”, following questions about possible tensions at the top.
The group is planning to elect a permanent leader at its first conference in September, where it will hold its official launch.
It also intends to drum up a manifesto through touring across the country. An MP involved said they would “deserve to fail if all we were was anti-Brexit” but insisted Britain’s departure from the EU was its most pressing challenge.
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