Bridget Phillipson, a Labour lawmaker, said that women in Parliament routinely received vile messages calling them, among other things, witches. She said that while Theresa May, the former prime minister, had kindled some anger at MPs during her leadership, the mood darkened when Johnson took office.

"It's treachery, treason, betrayal and, added to it, a healthy dose of misogyny," Phillipson said, describing the threats. "It's a deliberate strategy of Boris Johnson to fuel these divisions, to pit the public against Parliament and their politicians."

Long-serving MPs, veteran journalists and British historians said they had never seen a more noxious session in Parliament than the one that unfolded, when Johnson returned to face MPs after the Supreme Court ruled that his prorogation of Parliament had been "unlawful".

Opening Parliament on Thursday, Speaker John Bercow said the atmosphere the day before was "toxic" and "worse than any I've known in my 22 years in the House".

Bercow urged lawmakers to "treat each other as opponents, not as enemies."

In a bombastic performance, Johnson used terms such as "betray", "sabotage" and "surrender" to describe the manoeuvring of lawmakers who are getting in the way of his promise to leave the European Union, with or without a deal, on October 31. His opponents called him "unfit to serve" and shouted for his resignation.

We should not resort to using offensive, dangerous or inflammatory language for legislation that we do not like.

Labour MP Paula Sherriff

Tracy Brabin, the Labour politician who holds Cox's former seat, was among the MPs who took issue with Johnson's refusal to say the "Benn Act," as others did, to describe the legislation that requires him to seek a Brexit extension if he hasn't struck a deal with the EU by October 19. Instead, Johnson called it the "surrender act," "humiliation act" and "capitulation act."

Brabin said using those phrases suggested that those who disagree with Johnson are traitors, not patriots.

"Please, please, will he going forward moderate his language, so we will all feel secure when we are going about our jobs?" she pleaded.

In another exchange, Labour lawmaker Paula Sherriff said, "We should not resort to using offensive, dangerous or inflammatory language for legislation that we do not like."

Gesturing toward a plaque on the wall honouring Cox, she said: "We stand here, Mr Speaker, under the shield of our departed friend, with many of us in this place subject to death threats and abuse every single day. And let me tell the Prime Minister that they often quote his words 'surrender act, betrayal, traitor,' and I for one am sick of it. We must moderate our language, and it has to come from the Prime Minister first."

Johnson responded that he had "never heard such humbug in all my life".

Cox's widower, Brendan Cox, said there were reckless words on both sides of the debate. There was a vicious cycle, he told the BBC on Thursday, "where language gets more extreme, response gets more extreme ... The reason it matters is because it has real-world consequences.

"I was genuinely shocked by the willingness to descend to vitriol, because I think it does long-lasting harm," he said. "To have this debate descend into this bear pit of polarisation, I think it's dangerous for our country."

Rachel Johnson, the Prime Minister's sister, an author and politician who opposes Brexit, called her brother's remarks "tasteless".

She said: "I think it was particularly tasteless for those who are grieving a mother, MP and friend to say the best way to honour her memory is to deliver the thing she and her family campaigned against - Brexit."

She added that "my brother is using words like 'surrender' and 'capitulation' as if the people standing in the way of the blessed will of the people, as defined by the 17.4 million votes in 2016, should be hung, drawn, quartered, tarred, and feathered. I think that is highly reprehensible."

Nicholas Soames, an independent lawmaker and grandson of Johnson's idol, Winston Churchill, told broadcasters that Johnson was stoking further divisions in a country already deeply divided over Brexit.

"I believe the job of the Prime Minister, even under very difficult circumstances, is to try and bring the country together, and what the Prime Minister did yesterday was to drive it further apart."

Johnson didn't seem to be backing down. He continued to use "surrender bill" in a meeting with fellow Conservatives, according to reports on that later session. And when asked to respond in Parliament to an "urgent question" about his use of inflammatory language, he sent a junior minister.

Johnson has a reputation for blunt talk - and he regularly gets himself in trouble. During his first Prime Minister's Questions earlier this month, he was asked to apologise for his previous comments that compared Muslim women in Britain who wear a niqab to "letterboxes" and "bank robbers".

Analysts say Johnson knows exactly what he is doing. His strategy, they say, is to set up a "people versus Parliament" election, where the Latin-quoting, Eton and Oxford-educated Johnson would run against his fellow elites who stand opposed to "the will of the people".

The opposition parties don't want an election before October 31, the day Britain is scheduled to leave the European Union. But many think that an election could follow soon after.

Johnson says he seeks to cut a deal to leave the EU in orderly fashion. But his abrasive style seems almost designed to alienate MPs whose support he will need if he hopes to pass a withdrawal deal through the House of Commons.

Nicky Morgan, who serves in Johnson's cabinet, tweeted: "I know the PM is aware of & sympathetic about the threats far too many of us have received because I shared with him recently the threats I am getting. But at a time of strong feelings we all need to remind ourselves of the effect of everything we say on those watching us."

Amber Rudd, who served in Johnson's government until resigning recently, said she thought Johnson's language in Parliament was "dishonest and dangerous".

"I was in the tea room today, and lots of MPs were exchanging information about how they have seen the ramping up of threats against them," Rudd told ITV.

I didn't know Jo Cox but I'm certain this man is not fit to speak her name.

Nicola Sturgeon, Scotland's First Minister

Nicola Sturgeon, the first minister of Scotland, tweeted that "there's a gaping moral vacuum where the office of Prime Minister used to be". She added, "I didn't know Jo Cox but I'm certain this man is not fit to speak her name."

During the feverish debate, Bercow said he was aware that politicians from all parties had been threatened. "I would simply appeal to responsible colleagues in all parts of the House to weigh their words," he said.

Johnson said he knew there was a "lot of anxiety about language." He offered a remedy: "Let's make sure the very word Brexit is never heard in 2020. Wouldn't that be a fantastic thing?"