Jo Swinson accused Labour of “aiding and abetting” the SNP’s campaign for independence, as MPs clashed for the final time in Westminster before the general election.
The Liberal Democrat leader claimed Boris Johnson’s willingness to “accept a border in the Irish Sea” as part of his Brexit deal and Jeremy Corbyn’s openness to working with the nationalists had plunged the Union into “danger”.
The comments came as Ms Swinson told activists at the party’s election campaign launch she was now “a candidate to be prime minister”.
She said: “Our country needs us to be more ambitious right now – and we are rising to that challenge.
“It is not about the red team or the blue team, because on this issue they merge into one – both Labour and the Conservatives want to negotiate and deliver Brexit.
“I never thought that I would stand here and say that I’m a candidate to be prime minister, but when I look at Boris Johnson and Jeremy Corbyn, I am absolutely certain I could do a better job than either of them.”
Ms Swinson said Mr Johnson had “lied to the Queen, lied to Parliament and lied to the country” and “was not fit to to be prime minister”.
She then accused the Labour leader of failing to “give a straight answer on the biggest issues facing this country”.
Later asked if she agreed with former Lib Dem leader Nick Clegg’s assessment that Brexit had set the clock ticking on the end of the UK, she said: “I think it is fair to say that we’re in a dangerous time for our United Kingdom.
“It’s only the Liberal Democrats who are standing for the United Kingdom.
“You have the SNP obviously trying to break up the UK, you have the Labour Party aiding and abetting them in that and the Conservatives undermining the relationship between Northern Ireland and rest of the UK, so these are dangerous times”.
Aberdeen North SNP MP Kirsty Blackman dismissed the Lib Dem leader’s concerns, saying: “People and communities across Scotland have not forgotten the devastation Jo Swinson caused the last time she went into government with the Tories – pushing millions of families into poverty and inequality.
“That is a record she said she was ‘proud’ to be a part of.
“The Lib Dems have propped up the Tories before – and they would do it again at the drop of a hat.”
Meanwhile, Mr Corbyn used an event in the bellwether seat of Harlow, Essex, to argue that he was “listening to the whole country” on Brexit.
He said: “People sometimes accuse me of trying to talk to both sides at once in the Brexit debate, to people who voted Leave and Remain.
“You know what? They’re right. Why would I only want to talk to half the country?
“I don’t want to live in half a country. Anybody seeking to become prime minister must talk to and listen to the whole country.”
The comments came as Parliament was dissolved to allow for a five-week campaign before the election on December 12.