The Independent Group of MPs has been challenged by the petitions site Change.org about their plans to register as a political party called Change UK. Change.org has said it is seeking urgent advice to prevent the group using similar branding.

The breakaway group of former Labour and Tory MPs is to formally register as a political party in time for potential European elections, and has said the former Conservative MP Heidi Allen will be interim leader.

It is understood Change.org is seeking legal advice and contacting the Electoral commission. “This new party is using the language of Change.org. Our movement is one of 17 million people across the UK and we can’t allow ourselves to be hijacked in this way,” a company source said.

Soubry, one of the founder members of the group, accidentally called her new party Change.org in a speech in the Commons on Friday. She said the party would “change the face and direction of British politics – that’s why we call ourselves Change.org”.

In a statement on Twitter, Change.org said it was “totally independent of party politics” and said it was seeking advice on how to challenge the branding.

“It’s said that imitation is a form of flattery. But the movement we’ve built in the UK to win campaigns for ordinary people is ours – all 17 million of us,” the site said in a tweet.

The MPs in the breakaway party, who also include Chuka Umunna, Luciana Berger and Chris Leslie, formerly of Labour, and the former Tory Sarah Wollaston, said they had been advised to urgently register with the Electoral Commission in order to put up candidates.

Allen will be interim leader until the group holds its first conference in the autumn, where it will launch as a political party and officially elect a leader, it said in a statement.

The UK may have to hold European parliament elections on 23 May, with the close of nominations for MEP candidates on 24 April, if the government is forced to accept a long extension to article 50. The group will register as Change UK – The Independent Group.

Umunna said the group hoped to field a “substantial number of MEP candidates with backgrounds from outside politics” if the UK takes part in the European elections.

He said tens of thousands of people had signed up to support the group since the MPs defected from their parties last month.

“A new party will shake up the two-party system and provide people with an alternative that can change our country for the better,” Umunna said. “This is what Change UK will be aiming to do at any European elections if our application for registration is accepted in time.”

Allen said she wanted to attract support from across political backgrounds.

“Coming into the House of Commons from running my manufacturing business in 2015, I have seen with my own eyes how improved our political system would be if it harnessed the diverse skills and experience of our country,” she said.

“We in Change UK, as we hope to be known, don’t just dream about a fairer and better future for our country; we are determined to unleash it through hard work, passion and shared endeavour.”

All the 11 MPs in the group are backers of a second referendum on Brexit. Labour MPs who quit the party to join the group cited disagreement with both Jeremy Corbyn’s Brexit policy and his handling of antisemitism allegations made against party members.

Three Tory MPs joined the group two days after the Labour MPs defected, saying they believed their party was being held hostage by hard Brexiters.