Boris Johnson has refused to apologise in the face of criticism that he is inciting hatred against MPs, as he briefed his cabinet on preparations for a populist election campaign that will accuse his opponents of “surrender” to the EU.

In the face of widespread condemnation for his inflammatory rhetoric, the prime minister vowed to carry on referring to the Benn law against no-deal Brexit as the “surrender bill”.

He also declined to retract his comments about the murder of Jo Cox, after saying the best way to honour her memory was to “get Brexit done”.

'Abuse is virtually constant': female MPs speak about the threats they face Read more

“Obviously I’m deeply sorry for the threats that MPs face and I think it’s very important we look after them, particularly look after female MPs,” he told the BBC.

“The death of Jo Cox was an absolute tragedy which I think bought the House of Commons together in unison. But it’s also important to protect the right of MPs to speak freely in the House of Commons about important political matters and the fact of the so-called Benn Act is that it surrenders our powers.”

Tensions remained high in parliament after Wednesday night’s toxic and heated scenes in the House of Commons. A Labour MP, Karl Turner, confronted Johnson’s senior adviser, Dominic Cummings, over death threats received by MPs overnight following the parliamentary debate. Cummings responded: “Get Brexit done.”

At an event later, Cummings later doubled down on his suggestion that only carrying out Brexit would calm tensions.

“I know people on both sides of leave and remain who have had serious threats like that, and it is obviously bad. In the end the situation can only be resolved by parliament honouring its promise to respect the result.”

Johnson’s hardline language and tactics appeared to show he is increasingly focused on fighting an election, amid pessimism in the government and Brussels about the chances of reaching a new EU withdrawal agreement at the October summit. He told his cabinet that the phrase “surrender bill” was hitting home with voters and would form a key part of any campaign.

Play Video 2:43 Outrage as Boris Johnson dismisses dangers of inflammatory language as 'humbug' - video

With Labour refusing an election until no deal is off the table, Johnson is still insisting that he will not have to seek an extension from the EU if he fails to reach a deal.

Many opposition MPs are sceptical that there really is a way to circumvent the Benn law, but Sir John Major, the former prime minister, will warn on Friday that the government could seek a “order of council” to delay that legislation coming into effect.

“My fear is that the government will seek to bypass statute law, by passing an order of council to suspend the Act until after 31 October,” he said. “It is important to note that an order of council can be passed by privy councillors – that is government ministers – without involving the Queen.

“I should warn the prime minister that – if this route is taken – it will be in flagrant defiance of parliament and utterly disrespectful to the supreme court. It would be a piece of political chicanery that no-one should ever forgive or forget.”

No 10 declined to comment on the idea but there were signs that Johnson’s administration has not resiled from its combative approach to the law, which led to the supreme court ruling unanimously that the suspension of parliament was unlawful.

A cabinet source said Johnson is preparing to prorogue parliament again a few days before a Queen’s speech to set out his domestic agenda, with the current intention to stick to the original date of 14 October. The prime minister also intends to carry on with Conservative party conference next week as usual, while opposition MPs sit in parliament attempting to undermine his strategy.

Throughout the day on Thursday, Johnson took a defiant approach to the criticism of MPs who lambasted him for his language in the House of Commons, refusing to appear for a question in parliament on his rhetoric. Instead, he addressed Tory backbench MPs, telling them that he would carry on referring to the “surrender bill”.

Tory MPs said the mood of Johnson’s meeting had been “largely supportive” but others sighed or raised their eyebrows when asked how he had performed.

Nicky Morgan, the culture secretary, sent a pointed tweet saying that a “time of strong feelings we all need to remind ourselves of the effect of everything we say on those watching us”, although she cushioned it with a claim that the prime minister was sympathetic about threats received by MPs.

However, Johnson was criticised more directly by his own sister, Rachel, who told Sky News: “I think it was a very tasteless way of referring to the memory of a murdered MP, murdered by someone who said ‘Britain first’, of the far-right tendency, which you could argue is being whipped up by this sort of language.

Play Video 1:11 Boris Johnson's sister says his Jo Cox remarks were 'tasteless' – video

“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 17.4m votes in 2016 should be hung, drawn, quartered, tarred and feathered. I think that is highly reprehensible language to use.”

Johnson was also taken to task directly by Jess Phillips, the Labour MP, in the voting lobbies over his hostile rhetoric, on the same day a man was arrested outside her office.

The prime minister later acknowledged that there was a “reasonable anxiety” that MPs in other parties may be less willing to vote for any Brexit deal because of the divisive nature of Wednesday’s debate. However, some Labour backbenchers are still willing to consider what is on offer if there is a chance of resolving Brexit.

Stephen Kinnock, who is a member of the MPs for a Deal group, said: “Johnson’s language, tone and behaviour are not helpful but we must not allow ourselves to be distracted from the number one priority which is to secure a deal that protects … our constituents so that we can then begin to reunite our deeply divided country.”

When asked at Thursday night’s event in London whether Johnson was under pressure to deliver Brexit, Cummings said: “We are not under pressure, the referendum was pressure. The referendum was difficult. This is a walk in the park compared to the referendum. We are enjoying this, we are going to leave and we are going to win.”