JO Swinson has ruled out a second coalition with the Tories under Boris Johnson, branding him a “disaster” for the country and unfit to be Prime Minister.

The new Liberal Democrat leader, who was a minister in the 2010-15 LibDem-Tory coalition under David Cameron, said she was “crystal clear” that her resolutely Remain party would not work with a government lead by the arch Brexiter.

Mr Johnson has an effective Commons majority of two which could be reduced to one in a byelection next month, making his premiership precarious from the outset.

However Ms Swinson, who replaced Sir Vince Cable as UK leader of the LibDems on Monday, said Mr Johnson would get no support from her party’s 12 MPs.

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She said: “Boris Johnson has finally got his hands on the keys to Number 10, but he has shown time and time again that he isn’t fit to be the Prime Minister of our country. Whether it is throwing people under the bus or writing a lie on the side of one: Britain deserves better than Boris Johnson.”

Ms Swinson also told the BBC that Mr Johnson’s “do or die” plan for Brexit by October 31 was disastrous, and said it was telling that in his acceptance speech he had offered little detail, merely “the bluster and bravado of just saying it’s going to be fine”.

She said: “This is a man who really only cares about one thing - and that’s Boris Johnson. This is not a man who is seriously trying to unite the country. In fact, this is not a man who is serious at all.

“These are difficult times. We’re effectively in a national state of crisis with Brexit. If you look around the world, from Iran to Hong Kong, these are times for somebody that will address these issues with seriousness, not somebody who’s pretending to try and be a rather poor stand-up comic.”

She said an early general election was a live possibility, and her party was ready.

She said: “I would welcome that because I think Boris Johnson is a disaster for our country, both domestically and indeed on the world stage, and therefore I welcome the opportunity to take him on in a general election.”

Ms Swinson ruled out a pact with Labour leader Jeremy Corbyn in a hung parliament, saying he was a Brexiter who could not be trusted on a People’s Vote.

The 39-year-old said that if Leave won a People’s Vote she would “absolutely respect” the result, arguing that no-one voted for a “specific Brexit deal” in the first EU referendum.

She said “the rules of politics are being rewritten” and she hoped to get a majority in the House of Commons for a People’s Vote with the support of Tory ministers and MPs who have ruled out serving under Mr Johnson in opposition to his Brexit stance.

Ms Swinson, the MP for East Dunbartonshire, whose leadership role depends on the Union, also said she did not want Holyrood empowered to hold a new independence referendum.

Nicola Sturgeon, who has said she wants a vote on the constitution in the second half of 2020, is expected to ask Mr Johnson for referendum in the coming months.

Mr Johnson’s predecessor, Theresa May, repeatedly refused to grant a so-called Section 30 order, saying “now is not the time” to revisit the No result of 2014.

Ms Swinson told BBC Scotland the Yes prospectus had been “comprehensively rejected”, whereas a People’s Vote was a “way out of this current mess and crisis”.

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She said: “Chaos in our politics is stopping us from moving on and addressing the real issues our country faces, so there is actually a way out of chaos.

“If you were to have an independence referendum in Scotland, that is layering chaos on top of that already volatile and difficult situation.”

Confirming she would refuse a referendum if she was PM, she said: “There is no mandate to have that. The SNP have put forward this idea of Indyref2 at subsequent elections and they have lost seats and lost votes.

“In Scotland, most people want Scotland to be in the UK and the UK to be in the EU, and the Liberal Democrats are the only party that are arguing for that position.”

Scottish LibDem leader Willie Rennie added: “When faced with a choice of Boris Johnson or Jeremy Corbyn it’s pretty obvious that Jo Swinson should be in charge.”