Rachel Johnson, a journalist and Remain supporter, said her brother was using the despatch box as a “bully pulpit” in a “strongman gambit” to “whip up” support among Leave voters.
Following the public criticism from his sister, Mr Johnson showed the first sign of remorse after the House of Commons became a hotbed of anger and vitriol following his humiliating defeat by the Supreme Court over the suspension of Parliament.
In a tacit admission that he needed to start to show a more conciliatory tone if he hoped to get a new deal passed by the House of Commons next month, the Prime Minister accepted that he had to “reach out across the House to get Brexit done”. He added that he was “sorry” that MPs were receiving threats, something he said he “deplores”.
Criticism of 'bully' PM
His tone contrasted with the bullish stance he had adopted since Tuesday’s court verdict, which ruled that he had acted unlawfully in proroguing Parliament for five weeks.
Mr Johnson provoked outrage in the House of Commons on Wednesday evening over his repeated use of the words “surrender” and “betrayal” to describe MPs who had tried to block a no-deal Brexit.
He also angered MPs when he said they needed to “honour the memory” of the murdered politician Jo Cox to “get Brexit done” and that the way to ensure parliamentarians were “safe” was to deliver it on 31 October.
Paula Sherriff, the Labour MP for Dewsbury, accused the Prime Minister of “inciting hatred” against MPs with his language.
Rachel Johnson echoed that sentiment when she told BBC Radio 4’s World At One programme: “What we saw yesterday and today in the Commons is a very divided country.
“But to say that Parliament is at fault is not helpful, because Parliament is also divided and it’s reflecting the division in the country.”
She added: “I love him very much and he is a different person in the Commons” and that the tactic was the “kind of strongman gambit that has been proved to work”.
Ms Johnson also said she did not know who was behind Mr Johnson’s strategy – whether it was his senior aide Dominic Cummings or “people who have invested billions in shorting the pound or shorting the country in the expectation of a no-deal Brexit”.
In a series of interviews with regional BBC programmes, the Prime Minister defended his use of the word “surrender”, but added that it was a “reasonable anxiety” that his language was losing him votes in the Commons.
He said: “Tempers need to come down, and people need to come together because it’s only by getting Brexit done that you’ll lance the boil as it were of the current anxiety and we will be able to get on with the domestic agenda.”
He added: “I absolutely deplore threats to MPs… 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.
“The death of Jo Cox was an absolute tragedy which I think brought 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…
“There’s all sorts of language that people use of a metaphorical kind, or indeed literal kind, that is very abusive. And that, I agree, can sometimes stray too far.”