Boris Johnson faced a grave threat to his control of parliament on Friday as he was warned that Conservative rebels could cross the House of Commons to foil Brexit in the aftermath of a byelection that reduced his working majority to just one MP.

Overnight, the Liberal Democrats’ Jane Dodds won a crucial byelection in Brecon and Radnorshire by a margin of 1,425, overturning the Tories’ previous majority of more than 8,000.

The result prompted immediate recriminations across the party. Conservative no-deal sceptics warned about the rapidly growing threat the government could face from the reinvigorated Lib Dems, while insiders blamed Theresa May’s administration for choosing a candidate who had already been ousted for expenses fraud.

One of the most prominent Conservative supporters of a second referendum told the Guardian on Friday he was actively considering defecting to the Lib Dems or sitting as an independent, a move that would leave Johnson at the helm of a minority government.

Dr Phillip Lee, the former justice minister, who first suggested he could quit the party in his own podcast, On the House, told the Guardian he was not alone among colleagues considering defecting or resigning if the government pursued no deal. “I have things to think about over the summer, but it is not just me,” he said.

“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.”

Dr Phillip Lee: ‘The Conservative party has morphed into something a lot different to what I joined in 1992.’ Photograph: Conservative party/PA

Although Johnson might be able to rely on Labour Brexiters and independents to vote for a deal, Lee suggested that the government could still be threatened by the many Conservatives in the party’s centre who had been alienated by the number of rightwingers in Johnson’s cabinet.

“At the moment Boris Johnson has a very difficult pitch to play and that has been made even harder by the formation of this cabinet,” he said. “There are increasingly people who think, ‘Even if my career is over, I can’t put my name to this.’

“I am doing my best to represent my constituents – I cannot think of a business in my patch that is enthusiastic about Brexit, let alone no deal. It is an odd situation for the MP being threatened with deselection for being on the side of virtually every business in his constituency. You never would have thought a Tory MP could be in that position.”

One former cabinet minister said they believed swathes of seats were now at risk, including Cheltenham, Chippenham, Guildford and even Surrey Heath, the seat of Michael Gove.

“Threatening no deal essentially hands our core seats across the south-west, south-east and south coast to the Lib Dems,” the ex-minister said. “We’ll lose tons of decent MPs. The Lib Dems will pick off lots of the big beasts in Surrey and have some spectacular gains. Worst of all the new northern core will never materialise – the Labour vote is tribal. It’s a suicidal vote strategy. I’m beginning to think Dominic Cummings is a Lib Dem sleeper agent.”

Other Tory MPs have pointed out privately that the majority of seats where the Lib Dems are in second place are held by Conservative MPs.

One former cabinet source pointed to warnings given by several cabinet ministers who have departed, including Jeremy Hunt, David Gauke and Rory Stewart, about the electoral threat of the Liberal Democrats and the need to attract moderate Tory voters in order for the party to have a functioning majority.

The defeat in Brecon and Radnorshire came amid new warnings from the Bank of England governor, Mark Carney, that the challenges posed by a no-deal withdrawal on 31 October should not be played down. “It may take a while to get to the sunlit uplands,” the outgoing governor told the BBC. “With no deal, the shock to the economy is instantaneous and instantly … you actually have businesses that are no longer economic.

“One would expect prices to go up. It’s reasonable to expect – the markets are absolutely clear on this – that in the event of no deal, the exchange rate would go down for a period of time. And the area of the economy where that instantly translates into prices – it’s at the forecourt of the petrol station and in fruit and veg.”

A senior Conservative source said there was no change to the government’s plans, regardless of the prime minister’s majority and any potential change to that in the coming months. “We have made clear we are leaving on October 31. The most recent polling of what happens to British politics if there is any more delay is really not pretty and it is not good for anybody,” the source said. “In terms of delivery, we are focused on that and there is not a single change in our planning.”

The party is likely to begin to target potential Conservative-Lib Dem swing voters with messaging about the new government’s domestic priorities, as well as emphasising the unhealthiness of continued impasse at Westminster, one Tory source said. “This PM has a whole bunch of domestic priorities, things that voters really care about.

“The party has to send the message that voting Lib Dem is not just a matter of potentially ignoring a democratic vote to leave the EU – and that’s going to be a key message for us that senior figures have discussed – but it also means more uncertainty, more malaise, more gridlock.

“It means those domestic priorities don’t get delivered. And that’s the message to those Conservative voters who are moving away and why they should come back.”

The Lib Dems’ new leader, Jo Swinson, hailed the victory in mid-Wales as the party “winning and on the up” – a win that was partially down to a remain alliance, with the Greens and Plaid Cymru standing aside to give her party a clear run at the seat.

The selection of the former Tory MP Chris Davies to run again in the seat, after he was ousted in a recall petition over a conviction for expenses fraud, was the source of mass recriminations among Conservatives in Westminster, given that the Lib Dem margin of victory was relatively small.

Mark Wallace, the executive editor of ConservativeHome, tweeted that it was “impossible to avoid the conclusion that, with a new candidate not mired in controversy, the Conservatives probably could have held the seat, given that margin”.

One Tory campaign source said they believed there would have been a strong chance of keeping the seat with a few more days’ campaigning – and a different candidate. “Clearly, this candidate was not chosen by this prime minister – there are a lot of ‘what ifs’ that can be asked about this process,” the source said.

“Another week to campaign with both a reinvigorated government and prime minister would have seen a different result. We definitely squeezed the Brexit party very heavily.”

Steve Baker, vice-chair of the hard Brexit European Research Group, said the Brexit party had in effect helped to elect an anti-Brexit MP. “It is becoming obvious to all now that the Brexit party standing against the Conservative party would produce a massive own goal,” he tweeted.

The party, which is led by Nigel Farage, has shown no sign of wanting to enter a pact with the Conservatives thus far. Overnight, the Brexit party announced a number of candidates it would stand in target seats at the next election, including in crucial Labour-Tory marginals.

Candidates have been chosen in seats like Southampton Itchen, where the current MEP Alexandra Phillips will stand, the hyper-marginal seat of Dudley North, where Ian Austin quit the Labour party and sits as an independent, and the long-time Labour-Conservative battleground of Hartlepool.

Other notable candidates include the former independent mayor of Mansfield, who will stand in the traditional Labour heartland seat that was unexpectedly won by the Conservatives’ Ben Bradley in 2017, and the MEP and journalist Martin Daubney, who will stand in Ashfield, a marginal Labour seat targeted by the Tories where the MP Gloria De Piero has said she will stand down.