But while Jo Johnson called Theresa May's Brexit strategy "a failure of British statecraft unseen since the Suez crisis" and being bound by European Union rules with no say a "never-ending purgatory", he mocked his brother's suggestion that a no deal Brexit would be a realistic option, adding: "inflicting such serious economic and political harm on the country will leave an indelible impression of incompetence in the minds of the public".
"It was a false prospectus, it was a fantasy set of promises that have been shown up for what they were", he told the Today programme on Radio 4.
He told the BBC: "When we get the final deal, and it feels like that's not very far away, Cabinet ministers will have to look into their hearts and see whether or not they feel they can support it".
He added: "It's for each MP to come to his or her own view".
He added: 'This is one of the most momentous questions we will ever face in our political careers.
Johnson suggested that other senior Conservatives might be considering their positions, saying that "many are reflecting hard about the deal that's looming and how they will respond to it".
Mr Johnson insisted his resignation was not an attempt to oust the PM. "My priority is really just to do my best as a now backbench MP to try and encourage the country to pause and reflect before we do something that is irrevocably stupid", Johnson said.
Oil tanker rams into Norwegian frigate, leaving eight injured
Its crew of 137 had been evacuated, rescue leader Ben Vikoeren at the Joint Rescue Coordination Centre for southern Norway said. The frigate is believed to be the KNM Helge Ingstad , which took part in NATO's massive Trident Juncture 2018 wargames.
The EU insists that a deal must lead to either a soft Brexit for the entire United Kingdom, or a sea border between Great Britain and Northern Ireland.
Boris Johnson resigned as Foreign Secretary in July in protest at the Prime Minister's Chequers proposal, claiming it would leave the United Kingdom a rule taker from Brussels.
The former transport minister quit the government yesterday so that he can vote against what he rightly regards as the greatest humiliation since Suez.
Mrs May is also under pressure from the DUP over her Brexit plans, as well as Eurosceptic Tories.
"This would not be about re-running the 2016 referendum, but about asking people whether they want to go ahead with Brexit now that we know the deal that is actually available to us, whether we should leave without any deal at all or whether people on balance would rather stick with the deal we already have inside the European Union".
The Government has ruled out a second referendum.
"I think a second referendum would be divisive, but it wouldn't be decisive".