Chief Brexiteer and big brother Boris Johnson said the pair may not agree about Brexit, but were "united in dismay" over the UK's position. It was unclear whether others would follow Johnson out of government. May had been expected to rally her cabinet around her Brexit plan at a meeting originally due this week.
May issued an immediate rejection through her Downing Street spokesman: "We will not under any circumstances have a second referendum".
"We've been hurtling towards a blindfolded Brexit for too long, so it's about time that politicians hand back control to the people of this country by giving them the final say on Brexit - with the option to stay and lead in Europe", said Eloise Todd of the Best for Britain anti-Brexit campaign.
That wouldn't be an enormous surprise but it would be if it were a deal that May could force through Parliament, especially as this week the DUP, on whom May is dependant for majority support in the House of Commons, said they felt betrayed by plans for a customs border in the Irish Sea. "I've done so, if others feel that it's right for them to do so then good on them".
I'd say you couldn't make it up, except I think I said that about something Brexit-related last week (Arron Banks saying that he'd vote Remain in a second referendum, perhaps?) and the week before.More news: Trump joins Macron and world leaders at armistice ceremony
"The "new" idea of a United Kingdom customs arrangement does not appear to be a genuinely UK-wide offer but a GB offer and an NI offer badged as one - Northern Ireland in the EU customs territory and GB in a customs union", she wrote in the Belfast Telegraph.
He already expressed fears that the current "chaotic" Withdrawal Agreement being finalised would be "a awful mistake" in a blog, published yesterday to coincide with his departure. "What we can do is recognise the reasons why people voted Leave".
Leading Tory Brexiter Jacob Rees-Mogg rejected calls for a second referendum, but told the BBC he agreed with criticism of the deal.
Voting against the deal would mean leaving the European Union without any agreement at all, the government has warned.More news: Barcelona Bomb Out Dembele Over Training Ground Tensions
"The prime minister thanks Jo Johnson for his work in government".
She said: "Huge respect for Jo Johnson".
His older brother Boris resigned in July to protest May's Brexit plan - but Boris did so as a staunch support of a hard-line Brexit, while Jo Johnson backed the "remain" side during the June, 2016 referendum.More news: Veterans Day: Freebies, discounts and deals for our military