Boris Johnson has been warned over the fragility of his leadership after his working majority was reduced to just one by the Tories’ loss in the Brecon and Radnorshire by-election.
The new Prime Minister was told he may face defections or resignations from Conservative MPs who are in opposition to his hardline Brexit approach.
Meanwhile, Lib Dem leader Jo Swinson predicted further electoral alliances after Plaid Cymru and the Green Party stood aside in the mid-Wales constituency to boost the chances of a pro-EU MP succeeding.
Tory former justice minister Dr Philip Lee said he and colleagues were considering their next moves if the government pushed for a no-deal departure.
The MP told the Guardian: “I have things to think about over the summer, but it is not just me.
“There are a number of colleagues who are spending the summer reflecting on what is the right way for them to confront this no-deal scenario.
“Of course, it is difficult for all of us because we joined the Conservative party, but it has morphed into something a lot different to what I joined in 1992.”
Conservative former attorney general Dominic Grieve has previously said he would consider resigning the Tory whip if Mr Johnson led the party, while other pro-EU MPs could follow.
The Tories lost Thursday’s vote to the Lib Dems by a margin of 1,425 votes, in what was the quickest by-election defeat for any PM of the post-war period.
The ballot was triggered when then-MP Chris Davies was ousted by a recall petition after he admitted submitting false invoices for expenses.
But the Conservatives selected him to run again and he lost to Welsh Lib Dems leader Jane Dodds, whose party swung the vote nearly 12% from the Tories.
The victory was aided by the “Remain alliance” of anti-Brexit parties, and Ms Swinson predicted further pacts in future.
In a celebratory appearance alongside her new MP, Ms Swinson said: “I will fight to keep our country in the European Union, and we now have in Parliament one more MP who will fight to make that happen.”
The Lib Dem leader said she had exchanged messages with the Green and Plaid leaderships and told the BBC: “I think there will be more co-operation in future elections.”
Plaid Cymru’s Westminster leader Liz Saville Roberts also praised the alliance.
“The victory of Remain cross-party co-operation and Jane Dodds in Brecon and Radnorshire last night will give those in Downing Street plotting a snap general election pause for thought, because it shows they cannot take the public for granted,” she said.
The Tory loss will add to the PM’s challenges to steer Brexit through Parliament and may increase the chances of a snap general election.
Some Conservative commentators blamed the Brexit Party for splitting the “pro-Brexit vote” – as together their vote share amounted to 49%, more than the Lib Dems.
But the loss leaves the PM with the support of 319 MPs, including the DUP which props up his majority, while opposition parties now have 318.
