Labour and Conservative parliamentarians are anxious that the new breakaway group formed by Luciana Berger and Chuka Umunna has increased the chances of Theresa May calling an early election.

On the Labour side, MPs and peers were worried that the prime minister would be tempted to exploit a split in the opposition if more of their number defected to the new political group.

Stewart Wood, a Labour peer and political academic at the University of Oxford, said he believed the defections had “significantly raised the chances of a snap election being called”.

The former adviser to Ed Miliband said the prospect would be intensified if “20, 30 and even 50 Labour MPs think they can’t face the thought of standing on a manifesto for Jeremy Corbyn again”.

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At the same time, some Conservatives are increasingly concerned May could be forced into calling a snap poll if her slim majority is narrowed or wiped out by Tories resigning to join the Independent Group. Several Tory MPs told the Guardian they thought the chances of an election had increased in the last week.

The Conservatives insist they are planning for an election in 2022. But two Tory sources said their suspicions had been raised by invitations to dinners held by Adam Memon, director of research at the Conservative party, where they were asked what should be put in a general election manifesto.

The dinners, held at a central London restaurant away from Westminster, were attended by special advisers, No 10 policy officials and representatives of thinktanks, where attendees were invited to contribute their policy ideas.

Another source said a senior Conservative former minister had been charged with the drawing up of a manifesto in secret just in case there was a need to move quickly.

Asked whether a manifesto was being written, a Conservative spokesman said: “The next general election is in 2022. CCHQ’s role is to campaign to keep Conservatives in local and national government. Unsurprisingly, part of that involves planning and policy development – which continue as usual.”

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Polling by Sky data gave the first indication of what the public could think about a new independent grouping, even though it has no leader, manifesto or funding and is yet to form an actual political party. The research found support of 32% for the Conservatives, 26% for Labour, 10% for the independent grouping, 9% for the Liberal Democrats, 6% for Ukip and 4% for the Greens. However, this would still suggest May falling short of a majority.

Others believe May would be unwise to call an election given that the Conservatives could lose as many votes to a new centrist party as Labour.

Prof Rob Ford, an academic and Labour expert at the University of Manchester, said people should remember that the Social Democratic party, formed by MPs leaving Labour in the 1980s, took support from the Conservatives as well.

“If Tories are saying ‘ha ha’ to Labour about the Independent Group, then it doesn’t strike me that they’ve thought very deeply about the possible consequences,” he said.