Boris Johnson backed his brother's decision, saying: "We may not have agreed about Brexit but we are united in dismay at the intellectually and politically indefensible of the UK position". "There are many Conservative MPs who share Jo Johnson's serious concerns".
Theresa May's Brexit plans have come under intense pressure after a bombshell ministerial resignation amid grave warnings from the DUP that they may withdraw vital Commons support for the proposals.
The shadow foreign secretary, Emily Thornberry, has attempted to calm Labour dismay at Jeremy Corbyn's announcement that "we can't stop Brexit", by insisting the option of campaigning for a second referendum was still on the table.
May is expected to hold a cabinet meeting later this month in the hope of securing her ministers' support for her negotiating stance - and hopes to strike an exit deal with the European Union in the next few weeks.
The critique from Johnson underscores the travails that May faces in getting any Brexit divorce deal, which London and Brussels say is 95% done, approved by her own fractious party.
A Downing Street spokesman said: "The referendum in 2016 was the biggest democratic exercise in this country's history". They're questions about the Northern Ireland border, they're questions about the kind of regulatory framework in which we'll live.
"It has become increasingly clear to me that the withdrawal agreement, which is being finalised. even as I write, will be a bad mistake", he wrote in a stinging resignation statement.
In a statement published on the Medium website, the younger Johnson claimed the Brexit deal the Prime Minister is pushing for would leave the United Kingdom "economically weakened, with no say in the European Union rules it must follow and years of uncertainty for business".
Austria Accuses Colonel of Spying for Russia for Decades
Austrian ambassador to Russia Johannes Eigner leaves the Russian Foreign Ministry headquarters in Moscow on November 9, 2018. Russian Foreign Minister Sergey Lavrov said he had been "unpleasantly surprised" by the news from Vienna .
Premier League superstar signs a five-year deal amid Real Madrid links
Sterling has committed his future to City with the new three-and-a-half year contract extension worth around £275,000-a-week. Raheem has improved dramatically in the past two seasons and is now one of the Premier League's best attacking players.
Mazda teases its all-new 3 ahead of Los Angeles debut
In the end, the premium European automakers are trying their best to make their diesel engines silenced. A fresh teaser image proves that a Mazda 3 Fastback will be unveiled alongside the new hatchback.
At the party conference, Labour delegates left open the possibility of backing a second referendum if it could not force a general election.
Ministers have released a number of documents advising businesses and the public of what they need to do if there is no Brexit deal.
Former BBC journalist and current University of Kent chancellor Gavin Esler said: "This - from a hugely respected minister - is an important development".
"Brexit has divided the country".
Johnson - who backs Britain remaining in the European Union - said he is supporting calls for a second referendum on whether the country should leave the bloc.
Instead, Johnson said it would be proper to give the British public a chance to vote again because of the way the Brexit negotiations have evolved.
But Sir Keir said the Prime Minister's attempts to "threaten rather than persuade" would not work and Labour is prepared to reject the government's plans.