The Tory leadership frontrunner confirmed the option was on the table after Jeremy Hunt urged him to be “straight with the people” and accused him of getting “some important facts wrong”.
It came as a senior ally of Angela Merkel savaged the former foriegn secretary by saying he "invented stories" while working as the Daily Telegraph's Brussels reporter - and that he "has not changed".
Meanwhile, Labour leader Jeremy Corbyn came under huge pressure to sack Chris Williamson following a row over antisemitism - before the outcast Derby North MP had the whip suspended for a second time.
Mr Williamson, a close ally of Mr Corbyn, was first suspended in February after being recorded claiming Labour had “given too much ground” and been “too apologetic” in tackling antisemitism.
Theresa May met with Vladimir Putin at the G20 summit in Japan in a chilly encounter that will likely be remembered for the no-love-lost handshake the pair shared in front of press photographers.
The meeting followed the Russian president's claim that liberal values were now obsolete in the face of populist surges in the US and Europe.
See below how we covered the day's events
Good morning and welcome to our rolling coverage of the latest developments in the Tory leadership contest, Brexit and UK politics in general.
At last night's Conservative Party hustings in Bournemouth, Boris Johnson gave the clearest signal yet that he was prepared to suspend parliament to force through a no-deal against the will of MPs.
The former foreign secretary, who has previously skirted round the question, said it would be "absolute folly" to take the option off the table, describing it as "an essential tool of our negotiation".
He told party members he wanted to be the prime minister of a "representative democracy, a great representative democracy in which we believe in our elected representatives to take the right decision".
But challenged to categorically rule out the option, he said: "I'm not attracted to the idea of a no-deal exit from the EU but, you know, I think it would be absolutely folly to rule it out. I think it's an essential tool of our negotiation. I don't envisage the circumstances in which it will be necessary to prorogue parliament, nor am I attracted to that expedient."
Mr Johnson has previously pledged to take the UK out of the EU by 31 October "do or die" - though he described the chances of a no-deal Brexit as "a million-to-one against".
Before the debate Jeremy Hunt sent a letter to his rival suggesting he had "got some important facts wrong" and urging Mr Johnson to be "straight with people" over his Brexit plans.
Referring to Mr Johnson's claims that the odds of No Deal were "a million to one against", he wrote: "Those odds are wildly different to those given by those in the business of actually judging the odds (some bookmakers put the chance of No Deal at 2:1), so I fear that this statement flies in the face of reality."
Asked if he stood by the odds during Thursday's hustings, Mr Johnson replied: "Yes I do... Well I mean there's not a bookies... but I do."
The two candidates are still wrangling over taking part in further TV debates. Both are committed to take part in a head-to-head on ITV on 9 July, though it is expected many of the party's 160,000 members will have returned their ballots by that date.
Sky had scheduled a debate for this week but cancelled it when Mr Johnson refused to attend.
In his letter, Mr Hunt asked him to take part in a TV debate next Monday: "Over the past few days you have got some important facts wrong. I believe it is now more vital than ever that we conduct head-to-head debates this week before ballot papers go out."
Boris Johnson's suitability for the job of UK prime minister has been questioned by a key ally of German leader Angela Merkel, writes Jon Stone.
"It’s fun to talk to him – it’s really fun to talk to him, intellectual fun. But to run a country?”, asked Elmar Brok, a senior MEP from the chancellor's CDU party.
Following Mr Johnson's comments about pushing through a no-deal Brexit, the prime minister said her successor must work with parliament to secure a deal.
Ms May, who spent two years attempting to work with parliament without success, said she was not prepared to offer her unconditional support from the backbenches. She was speaking to the Independent on a trip to Japan for her swansong global summit.
Meanwhile Jeremy Corbyn is coming under increasing pressure to withdraw the whip from Chris Williamson following an antisemitism row.
The Derby North MP - a strong supporter of the Labour leader - was controversially readmitted to the party on Wednesday after being issued with a formal warning by a National Executive Committee (NEC) anti-Semitism panel.
It prompted a backlash from MPs, who demanded Mr Corbyn step in to withdraw the Labour whip. Since our story last night, the number of protesting MPs and peers has risen to more than 120.
Mr Williamson was suspended four months ago after saying the party had been "too apologetic" in the face of criticism of the way it dealt with anti-Semitism within its ranks.
Following his readmission, the MP tweeted: "I'd like to express my heartfelt thanks for the avalanche of goodwill messages from grassroots members. I can now focus on representing local people in Derby Nth and working for a Corbyn-led Labour govt to positively transform the lives of millions. Together anything is possible!"
It prompted a further backlash, with Jon Lansman, the founder of the grassroots Momentum group, responding by saying Mr Williamson had not shown "one iota of contrition". He tweeted: "Such contempt for the party's verdict! He has to go!"
Labour MP Jess Phillips also called for the whip to be withdrawn from Mr Williamson.
She told the BBC Radio 4 Today programme: "This is a matter of principle about racism, about the Labour Party being the party of equality, the party that is anti-racist. That reputation is slipping away from us every day."
In Japan, Theresa May has begun her meeting with Vladimir Putin at the G20 summit.
The pair shook hands but did not smile, reports Press Association.
They are holding their first formal bilateral talks since the poisoning of former spy Sergei Skripal, and his daughter Yulia, with nerve agent in March 2018.
Ms May is to tell Vladimir Putin that the two chief suspects in the Salisbury chemical weapon attack should be made to face justice, writes political editor Andrew Woodcock.
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