Theresa May has said that MPs will have a choice between her proposed deal with the EU – or no deal at all.
In an interview with Panorama, she is also critical of plans by Brexiteers to resolve the Irish border issue.
But ex-foreign secretary Boris Johnson claims the government’s failure to resolve the border question has led to a “constitutional abomination”.
A BBC-commissioned survey indicates more people across the UK think the impact of Brexit will be negative.
Speaking to the BBC’s Nick Robinson, Mrs May says that if Parliament does not ratify the Chequers plan “I think that the alternative to that will be having no deal”.
But Mr Johnson’s column in Monday’s Daily Telegraph criticises her strategy to leave the EU.
He says Mrs May’s Chequers plan “would mean for the first time since 1066 our leaders were deliberately acquiescing in foreign rule”.
He describes a backstop for the Irish border as “an attempt to annex Northern Ireland” by Brussels in creating a border down the Irish Sea.
Mr Johnson says the prime minister’s solution to the Irish border question in the Chequers plan would mean the UK “must remain effectively in the customs union and large parts of the single market until Brussels says otherwise”.