Boris Johnson has urged calm following a furious parliamentary session but refused to apologize after critics accused him of using language that led to death threats and abuse.

'Surrender bill' jibe fury

Johnson returned to a febrile House of Commons this week after his decision to prorogue parliament for five weeks was declared unlawful by the Supreme Court.

The prime minister challenged his opponents to call an election, while critics called on him to resign. Lawmakers subsequently engaged in hours of furious argument, with MPs hurling allegations of betrayal and abuse of power across the chamber.

The prime minister provoked particular anger by repeatedly describing a law that forces him to ask the EU for a Brexit delay unless he can strike a deal as the "surrender bill."

"I think it's fair enough to call the surrender act what it is," he told the BBC on Thursday, insisting the law damaged his negotiating position with Brussels.

The hardline language is widely seen as the latest part of a prolonged election campaign, with Johnson telling his Cabinet that the phrase "surrender bill" cuts through with voters. 

Calm urged, no apology

Johnson said threats to lawmakers had to be addressed but declined to apologize for his words and disputed suggestions his language was stoking abuse.

The anger had become so intense that Brendan Cox, husband of Jo Cox, an MP murdered days before the 2016 referendum, warned it could encourage violence unless politicians toned down their rhetoric.

When female Labour MP Paula Sheriff on Wednesday said she had had threats from people echoing the prime minister's rhetoric, Johnson sparked renewed fury by replying: "I have never heard so much humbug in my life."

His critics say such language is often used in threats of violence or worse received by lawmakers, particularly women.

"We do need to bring people together and get this thing done," Johnson told the BBC on Thursday. "Tempers need to calm down and people need to come together because it is only by getting Brexit done that you will actually lance the boil of the current anxiety."

Cummings in rare public remarks

Dominic Cummings, the controversial senior aide behind the prime minister's strategy, said on Thursday that the Brexit chaos was a "walk in the park" compared to the 2016 referendum.

Cummings, credited with masterminding the Leave campaign in 2016, has shied away from public comment since joining Johnson's team in July.

"To put your mind absolutely at rest on that we are not under pressure at all. The referendum was pressure, the referendum was difficult," Cummings said, according to the Daily Telegraph newspaper.

"This is a walk in the park compared to that. All the Vote Leave team, we are enjoying this, we are going to win, we are going to leave, don't worry."

Cummings, later confronted by Labour MP Karl Turner over death threats received by MPs, responded: "Get Brexit done."

(With input from Reuters)