Party members voted yesterday at the Lib Dem conference in Brighton to campaign to stop Brexit entirely if there is election.
But the party will seek a second referendum until that happens, with MPs currently blocking the Prime Minister’s attempts to go to the country.
It came as Boris Johnson heads to the continent for a crucial Brexit showdown with Jean-Claude Juncker.
Ahead of his first talks with the EU commission president, the PM insisted he is ‘passionate’ about getting a deal and hailed ‘signs of movement’.
Ms Swinson told GMB: ‘The government has gone away taken the result of that referendum and negotiated what that will mean for the British public.
‘What is on offer today is not what people were promised.’
Her position prompted Mr Morgan to ask: ‘Why should I accept your win?’
Ms Swinson clashed with Piers Morgan on GMB this morning, appearing live from the Lib Dem conference in Bournemouth
Prime Minister Boris Johnson (pictured left in Downing Street today and right in a ‘selfie’ video posted on Twitter) has said he believes ‘passionately’ that a new Brexit deal is possible before October 31
She continued: ‘Just to be crystal clear about this, our position in that election, when we go to the country and campaign… we will be saying if you elect a Liberal Democrat majority government, we will revoke Article 50.
‘If the people of this country then choose to elect a majority Liberal Democratic government, then we will do exactly as we’ve said during that election campaign and revoke Article 50. That’s the way democracy works.’
The motion passed overwhelmingly at the conference says a Liberal Democrat majority government would be ‘recognised as an unequivocal mandate to revoke Article 50 and for the UK to stay in the EU’.
The Lib Dems have experienced a surge under Ms Swinson, leaping upwards in the polls and securing a raft of defections from the Tories and former Labour MPs.
But the chances of them forming the next Government alone are slim.
Ms Swinson told GMB: ‘The government has gone away taken the result of that referendum and negotiated what that will mean for the British public’
Delegates at the party conference in Bournemouth yesterday voted overwhelmingly to support a motion to revoke Article 50 if the party takes power
Ms Swinson today also refused to deny that she would campaign to rejoin the European Union if Brexit happened.
In a second interview today, with the BBC, she said: ‘I absolutely am determined that we keep our place in the EU.
‘And I have not given up. I still believe we can stop Brexit.’