Boris Johnson is calling on his former cabinet colleagues to stage a mutiny over Theresa May's Brexit plan – but he admits "it will make little difference".
Writing in The Daily Telegraph, Mr Johnson says the PM is "on the verge of total surrender" to Brussels and her proposals are "a recipe for continued strife".
"We are on the verge of signing up for something even worse than the current constitutional position. These are terms that might be enforced on a colony," he adds.
Mr Johnson's stinging attack on Number 10 comes days after his brother, Jo, quit his role as transport minister.
:: Jo Johnson's resignation could signal real trouble for Theresa May
Jo Johnson said the PM's Brexit proposal was a "failure of British statecraft on a scale unseen since the Suez crisis".
Over the weekend, promises of a deal with the EU collapsed with both sides stuck on the terms of the Irish backstop.
0:47The UK and the EU want to avoid border checks between Northern Ireland and the Republic but cannot agree how that would work.
Brexiteers insist any arrangement should be time limited, with the UK able to say when it breaks away.
Commons leader Andrea Leadsom warned that if there is no get-out clause "I very much doubt we could get it through parliament".
:: Military could be used in the event of a 'no-deal' Brexit
The EU rejected Mrs May's plan for a mechanism which would allow the UK to leave such an arrangement.
Mr Johnson, who resigned as foreign secretary in July over Mrs May's Brexit strategy, writes in his column: "Even if we agree with the EU that the UK must have a unilateral break clause, so that we can go our own sweet way at a time of our own choosing, it is irrelevant because the programme and ambition of the government is to remain in captivity, to stay in our cell even if we are given the theoretical key to escape."
With no deal reached, time is running out to secure a special EU summit before the end of the month.
An extra cabinet meeting had been pencilled in this week but that's looking highly unlikely.
Mr Johnson warns MPs not to surrender to the "scare tactic" approach of the PM when she speaks of the "chaos of no deal".
0:26Downing Street is preparing for a difficult week, with more resignations a growing possibility.
Reports suggest a further four Remain-voting ministers are considering their positions.
Even if the PM agrees a deal with the EU and her cabinet, getting it through parliament remains a challenge.
More from Boris Johnson
With the DUP and hardline Brexiteers prepared to vote down any deal, the parliamentary arithmetic is looking increasingly difficult.
The government insists a deal can still be done, but Downing Street knows the slipping timetable leaves little time to achieve it.
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