Jo Swinson has ruled out supporting Jeremy Corbyn as prime minister in the event of another hung parliament, even if he promises to hold another Brexit referendum.

The Liberal Democrat leader “absolutely, categorically” dismissed suggestions that her party could help the current Labour leader into No10, insisting he was “not fit for the job or prime minister”.

Launching her party’s election campaign in Westminster, Ms Swinson insisted that she was campaigning to be prime minister and dismissed questions about potential support for other parties.

Asked whether she could help Mr Corbyn cobble together a Commons majority if the 12 December election delivers a hung parliament and he promises a Final Say vote, Ms Swinson said: “I am absolutely, categorically ruling out Liberal Democrat votes putting Jeremy Corbyn into No10.

“On so many grounds, Jeremy Corbyn is not fit for the job of prime minister.

She added: “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.”

Continuing her attack on Mr Corbyn, Ms Swinson said the Labour leader’s “complete and utter failure to root out antisemitism in his own party” was “a total dereliction of duty when it comes to respecting the value of equality”.

She added: “Any party leader in that situation should be examining their conscience about what is happening in their party and even now he does not do it – he dismisses that the problem is even there. He is not fit to be prime minister of our United Kingdom.”

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Ms Swinson refused to be drawn on the circumstances in which she would agree to work with one of the bigger parties.

However, after speaking the launch event, Liberal Democrat MP Chuka Umunna left open the door to his party working with one of the main two parties if they were to change their leadership.

He told The Independent: “The two main parties as currently configured are not fit for office. We are going out to be the biggest party in the House of Commons it’s up to people to decide what happens after that.”

Ms Swinson insisted people were “wrong” to suggest that the Liberal Democrats should not be aiming to form the next government.

She said: “Our country needs us to be more ambitious right now, and we are rising to that challenge because this choice is about the future of our country for generations to come.”