Jo Swinson has accused Labour of “aiding and abetting” the SNP’s bid to break up the UK - but would not rule out supporting the next Labour leader if they promised a second referendum on Scottish independence.
Promising a “seismic change” in British politics if voters backed the Liberal Democrats, Ms Swinson ruled out ever supporting a government led by Mr Corbyn, but left her options open if the Labour leader was replaced.
"We, Liberal Democrats, will stop Brexit," the Lib Dem leader promised, and said she was "absolutely certain" she would do a better job as Prime Minister than either Boris Johnson or Jeremy Corbyn.
Ms Swinson said “stranger things have happened” than the party vaulting from fourth place in the Commons to form the next government, citing the SNP landslide in 2015 when they nationalists went from six to 56 MPs.
The Lib Dem leader said it was a "dangerous time" for the United Kingdom, both in terms of Scotland’s future and the Brexit deal proposed by Boris Johnson, which creates a customs border in the Irish Sea.
She said she will "never stop fighting" for the United Kingdom, declaring: "I will not let people wreck it."
Ms Swinson said she is "absolutely categorically ruling out" Liberal Democrat votes putting Mr Corbyn into Number 10, highlighting the risk of the Labour leader having responsibility for the UK’s nuclear arsenal.
"On so many grounds, Jeremy Corbyn is not fit for the job of Prime Minister. On the biggest issue of the day, he has prevaricated and will not give a straight answer.
"Even now if you ask him whether he is Remain or Leave he will not tell you how he would vote.
"His plans for the economy would take us back to the 1970s. I believe he would be a threat to our national security."
However, when asked if she would rule out working with any Labour leader who opened the door to a second independence referendum, Ms Swinson declined to do so, saying: “Liberal Democrats are clear we are absolutely against having a second independence referendum.
“It is yet another reason why Jeremy Corbyn should be anywhere near Number 10, because he is made clear that he is not averse to having a second independence referendum.
“I know that many people within the Labour Party have a wish that their party was different and that somehow it’s going to change, and there will be some great new Labour hope.
“And I do not see the challenge, or how that will change significantly even if leadership might change the direction, I think it’s very unlikely to. So that’s pretty hypothetical.”
Andrew Bowie, the Conservative MP for West Aberdeenshire and Kincardine said: “As ever with the LibDems, Jo Swinson simply cannot be trusted to keep her word.
“She claims to oppose a second independence referendum, but then failed this morning to rule out backing a Labour leader who was proposing one.
“This isn’t a ‘hypothetical’, as she claims – it’s a very real prospect and all pro-Union parties in Scotland need to be on guard against it.”