Liberal Democrat leader Jo Swinson has urged the so-called “Gaukeward squad” of pro-European Tory former ministers to “stand up and be counted” on Brexit – because she fears not enough of them will try to stop Boris Johnson crashing the UK out of the EU without a deal.

Ms Swinson said the coming weeks represent a “big test” for Conservative MPs who recognise the risk of no deal but have so far held back from rebellion.

Without naming names, she made clear that the group included not only those walked out of the cabinet rather than serve under Mr Johnson (Philip Hammond, David Gauke and Rory Stewart) but also others who took ministerial posts despite previous misgivings about EU withdrawal (Amber Rudd and Nicky Morgan).

Speaking to The Independent less than two weeks after being elected Sir Vince Cable’s successor, she confirmed that she has been in contact with Tory former ministers who quit on Mr Johnson’s election. She has also held discussions with potential defectors who could eliminate his single-seat working majority at a stroke by crossing the floor in the run-up to the scheduled date of Brexit on 31 October.

With pro-EU Tories such as Phillip Lee and Margot James openly mulling quitting the party over Mr Johnson’s leadership, many in Westminster expect defections timed for maximum impact during conference season.

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Ms Swinson confirmed she has been in discussion with “people who share our values”, though she dismissed suggestions that she was holding out the offer of a peerage as a lure to potential switchers.

Asked whether she was planning to unveil defectors in an “autumn surprise” bombshell at September’s Lib Dem conference in Bournemouth, she would say only: “I’m hopeful to grow the parliamentary party further.”

Meanwhile, anti-Brexit MPs from all parties are continuing to hold talks about possible parliamentary mechanisms to stop the new prime minister forcing no-deal through on Halloween. They also want to establish the option of a Final Say referendum.

A series of crunch divisions – including a possible no-confidence vote to bring down the government – is expected when the Commons returns from its summer break in September.

And with Mr Johnson’s working majority reduced to just one by Lib Dem victory in Thursday’s Brecon and Radnorshire by-election, the result of the votes on which the country’s future hangs will rest on the decisions of a few wavering MPs.

Ms Swinson said: “I think it’s incumbent on people who in good faith have tried to get a deal through to recognise now that that is not where the current prime minister is heading. They are now faced with the prospect of having to push through a no-deal Brexit which they know will be a disaster and they need to put the national interest ahead of their personal and party interest.

“I’m confident that some will stand up and be counted, but I don’t think there is any guarantee that enough will. So it is important that every MP realises the power of the vote that they have.

“A lot of Tory MPs have to be critically examining their consciences, particularly some who have been very critical about no-deal and have now accepted ministerial roles. I think they have to think long and hard about what path they are trying to set this country on.

A force to Brecon with: by-election victor Jane Dodds stands behind Swinson (Getty)

Speculation is rife that Mr Johnson, buoyed by a “Boris bounce” for Tories in the polls, may call an early election to secure the majority he needs to get his Brexit plans through parliament.

But Ms Swinson said she had no fear of a snap poll, in which she said her party could potentially win “hundreds” of seats and become the largest group in the Commons on a Stop Brexit platform.

She has already ruled out a coalition with Mr Johnson or Labour’s Jeremy Corbyn in such circumstances, but said she was ready to explore options for a government of national unity with centrist pro-European MPs from all sides of the House.

“Absolutely there are other people in other parties who we have already worked with on the issue of Brexit, where it might be possible to work on a broader coalition across different parties or a government of national unity-type scenario,” she said.

“There are like-minded people who share our values who we could be working with, but a strong Liberal Democrat party needs to be at the heart of that.”

Having gained her first by-election scalp less than a fortnight after becoming leader, Ms Swinson was bullish about Lib Dem prospects, telling The Independent: “There’s no limit to my ambitions. The message from the polling we have is that hundreds of seats are in play for the Liberal Democrats now.”

Tory MP Phillip Lee has publicly questioned his own future in the party (Parliament Live)

Ms Swinson said Brecon – where Lib Dems unseated the Tories thanks in part to Greens and Plaid Cymru standing aside in a “Remain Alliance” – had “stopped the ‘Boris bounce’ within days of him taking over”.

She said she was open to future co-operation with other Remain parties, but insisted this would be on a “bespoke” basis, depending on circumstances in individual seats, rather than in a formal pact. And she noted that in “the vast majority” of cases, the Remain candidate best placed to win would be a Lib Dem.

Despite Mr Corbyn’s recent move towards a clearer pro-referendum stance, Ms Swinson held out no prospect of Lib Dems standing aside to help Labour beat Tories in seats such as Dover – where MP Charlie Elphicke faces sexual assault charges, which he denies – or Uxbridge, where there have been calls for a Remain alliance to kick Mr Johnson out of the Commons.

“If you want to be able to have an election where voters have a chance to back someone who will try to stop Brexit, clearly the Liberal Democrat candidate will fit that bill,” she said. “It’s not at all clear that a Labour candidate will.”

While polling suggests Mr Johnson is winning supporters back from Nigel Farage’s Brexit Party, Ms Swinson said the shift to the right he has executed to do so is driving “One Nation, small-l liberal Conservatives” towards her party.

Jo Swinson says she is 'absolutely over the moon' at result 'as the first woman to lead our party'

“The old rules of politics are being rewritten,” she said. “They are changing. I am a candidate to be the next prime minister and I look at Boris Johnson and Jeremy Corbyn and I am determined that our country has a better alternative. I could do a better job than either of them.”

Accusing the prime minister of “reckless irresponsibility” over the potential damage no-deal Brexit could do to the economy, public services and the NHS, she said: “He doesn’t really care, Boris.

“His whole approach is characterised by what is in the interests of Boris Johnson. If it gets him elected leader of the Conservative party, it’s fine by him. People are going to think twice.”

Liberal Democrats are on an election footing, raising funds and ramping up campaigning in expectation of an early poll, she said.