Rachel Johnson has said her brother - the Prime Minister - was “tasteless” for his comments linking a murdered MP to Brexit.
Boris Johnson was widely condemned after he rejected calls to temper his language and instead made inflammatory comments over Jo Cox, the Remain-backing MP who was killed by a far-right extremist in 2016.
On Wednesday evening during a heated session in the Commons, Mr Johnson said the best way to honour the late Mrs Cox was to "get Brexit done”.
Mrs Johnson said Boris had used “reprehensible language” during the fiery Commons showdown.
Speaking earlier this afternoon, the PM’s sister made further extraordinary comments, claiming his actions could be being influenced by “people who have invested billions in shorting the pound”.
She told Sky News’ ‘The Pledge’ show: "A lot of this language was initiated in the tabloids, because we had [headlines] like 'crush the saboteurs', we had the judiciary and remain MPs being 'enemies of the people', words like collaborationist, betrayal.
"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.
"I hope today in the Commons there will be some sort of deal on all sides that this sort of thing is utterly dialled down. It serves no purpose."
Speaking of her brother’s reference to Cox, 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.
"It was a very tasteless way of referring to the memory of a murdered MP, who was murdered by someone who said 'Britain first', obviously of the far-right tendency, which is being whipped up by this sort of language."
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The episode is due to be aired on Thursday night.
Downing Street has declined to apologise for the Prime Minister's words in the Commons.
Speaking earlier in the day on the BBC, the Prime Minister’s sister also hinted that such was the difference in her brother’s behaviour at the dispatch box to normal, it suggested he may be being adversely influenced of others.
She said: “He is a different person when he is speaking in the Commons. It’s not the brother I see at home - it’s a different person.
“So it raises the question: is it a deliberate strategy to raise the tempo so much that people feel they’ve had their democracy stolen, or betrayed, and whips up the base into believing that if they go to his strong man image, then they will get what they want.
“We are seeing an executive who is so keen to deliver Brexit in any shape or form to get the country out of the EU, they will do anything to justify that end.”
When asked if Mr Johnson’s personality change was down to the role of advisers, Mrs Johnson said: “It could be Dominic Cummings advising the PM to be extremely aggressive.
“It could also could be from, who knows, people who have invested billions in shorting the pound, or shorting the country, in the expectation of a no-deal Brexit.”
Yahoo News UK has approached Downing Street for comment.