Parliament can no longer block a no-deal Brexit, the health secretary has said.
During his bid for the Tory leadership, Matt Hancock said no deal was "not an available choice" to the next PM, as MPs "will never allow it to happen".
He told the BBC he had now changed his mind because they had a chance to block it in a series of votes last month, but failed to muster the numbers.
Jeremy Corbyn said Labour would do all it could to stop no deal.
But Downing Street insisted the UK would be leaving on 31 October "whatever the circumstances", whether a deal had been agreed with the EU or not.
"Politicians cannot choose which votes to respect. They promised to respect the referendum result and we must do," No 10 said.
Asked whether Mr Johnson would respect the outcome of votes in Parliament - including a vote of no confidence in his government - the spokesman refused to be drawn, calling it a "hypothetical" question.
MPs have repeatedly rejected the agreement Theresa May reached with Brussels, but have so far failed to coalesce around an alternative.
In a no-deal scenario, the UK would immediately leave the EU with no agreement about the "divorce" process, overnight exiting the single market and customs union - arrangements designed to help facilitate trade.
Opponents say it would damage the economy and lead to border posts between Northern Ireland and the Republic - but other politicians argue any disruption could be quickly overcome.
On 12 June, the Commons rejected a motion tabled by Jeremy Corbyn, which would have allowed MPs to take control of the parliamentary timetable in the autumn to stop no deal going through.
Mr Hancock told BBC Radio 4's Today programme it was that vote which prompted him to change his mind.
"I thought that would go through and in fact, the government won by 11," he said.
"I now don't think it can (stop no-deal). I thought that it could and the votes went differently to what I anticipated. When the facts change, sometimes even as a politician you have to change your mind."
Mr Hancock's comments echo a warning reportedly made by the prime minister's senior adviser, Dominic Cummings, that MPs have left it too late to stop no deal.
But Dominic Grieve, one of the most prominent pro-European Conservative MPs, told the BBC Mr Cummings was a "master of misinformation", and there were a number of options still left for MPs wanting to block no deal.
They include bringing down the government, via a vote of no confidence, and setting up a new government in its place, he said.
If Mr Johnson loses a vote of no confidence and it becomes clear that another potential prime minister could command the confidence of the Commons, the convention is that he would be obliged to resign.
However, Catherine Haddon, from the Institute for Government think tank, said this was "untested territory", adding: "He could say: 'No, I'm staying as prime minister and we're having a general election.'"
The date of an election is set by royal proclamation on the advice of the prime minister, so he could choose a date after 31 October and press on with Brexit in the meantime.
Speaking on a visit to Whaley Bridge to see efforts being made to shore up a damaged dam, Mr Corbyn said Labour would "do everything to stop no deal, including a no confidence vote at the appropriate, very early, time to do it".
He said Mr Johnson appeared to be "trying to slip no deal through, slip past Parliament and slip past the British people", and that was "not acceptable".