The Liberal Democrat leader, Jo Swinson, has ruled out entering into a coalition with the Conservatives or the Labour party if a general election delivers a hung Parliament.
Ms Swinson insisted that neither Boris Johnson nor Jeremy Corbyn was “fit to be our prime minister”.
She said: “I’m not going to support Boris Johnson or Jeremy Corbyn, They are not up to the job.”
And she said she wanted the party to win more than 300 seats in an election.
To secure a majority in the House of Commons, a party has to win more seats than all the other parties put together in a general election. At the moment, that means winning at least 326 seats – more to ensure a comfortable majority.
Ms Swinson dismissed the view that the Liberal Democrats were unlikely to win more than 300 seats.
“I reject this suggestion that you go into a general election campaign, particularly in these volatile political times and somehow people have to accept they don’t have a genuine choice.
“People do have a genuine choice and they do not have to choose between Boris Johnson and Jeremy Corbyn because frankly that choice is not good enough. Neither of those men is fit to be our prime minister.”
Ms Swinson, who has two children, added: “I’m not prepared to let my kids’ future be sold down the river just because the kind of previous rules of the way politics was done somehow have to apply.”
The Liberal Democrats currently have 18 MPs, including defectors from other parties including Chuka Umunna, Luciana Berger and former Conservative minister Sam Gyimah.
In recent times, the number of seats held by the party has peaked at 62 seats in 2005.
Questioned on the possibility of having to choose between supporting Mr Johnson or Mr Corbyn, she said: “I’m not going to support Boris Johnson or Jeremy Corbyn to be prime minister because they’re not up to the job. Our country deserves better.”
Asked to clarify that she would not support either man in the event of another hung Parliament, she replied: “Absolutely. They’re not up to it.”
On Sunday, Lib Dem members voted overwhelmingly in favour of a manifesto pledge to revoke Article 50 – the formal process of leaving the EU – if they came into power with a majority government.
Ms Swinson said: “As a party that wholeheartedly believes that our best future is within the European Union, we need to give the British people the chance to vote for that by saying that if you elect a Liberal Democrat government we will stop Brexit by revoking Article 50.”
And she revealed that she had had conversations over the weekend with further potential defectors because so many people were “unhappy” in the Labour Party and the Conservative Party.