A retweet by Culture Secretary Nadine Dorries, showing Rishi Sunak stabbing Boris Johnson in the back, has been branded “dangerous” by a fellow Conservative MP.

The picture has the prime minister and the former chancellor’s faces imposed onto those of Roman leader Julius Caesar and one of the men who conspired against and killed him, Brutus.

Greg Hands, who is backing the former chancellor in the Tory leadership race, told Sky News the social media post was “appalling”, especially after the fatal stabbing of Southend MP Sir David Amess in his constituency last year.

“It is not even a year since the stabbing of Sir David… so I think this is very, very bad taste, dangerous even,” he said.

“I do find it distasteful and I do find it, less than a year after the stabbing of our colleague, in very, very poor taste, even verging on dangerous.”

But an unnamed ally of Ms Dorries, who is backing Liz Truss in the leadership race, said: “It’s quite obviously a satirical image of Brutus and Caesar which has been clearly photoshopped to provide political commentary.

“There were similar cartoons involving [Michael] Gove in 2016. Some people of course will want to be wilfully offended.”

The culture secretary has been a staunch defender of the prime minister, and a fierce critic of those who contributed to his downfall.

It is not the first time she has caused controversy on social media, tweeting last week about the price of Mr Sunak’s suits, compared to the foreign secretary’s earrings from Claire’s Accessories.

She also wrote a damning piece in Saturday’s Daily Mail, accusing Mr Sunak of “planning a coup for a very long time” and saying she commented on his dress sense to “alert Tory members not to be taken in by appearances in the way that happened to many of us who served with the chancellor in Cabinet”.

Ms Dorries added: “The assassin’s gleaming smile, his gentle voice and even his diminutive stature had many of us well and truly fooled.”

But Mr Hands said the increasingly bitter contest to become the next Tory leader – and prime minister of the UK – needed to be “fought on the issues and on the leadership qualities” of the two candidates, rather than insults.

Former Northern Ireland secretary Brandon Lewis, who is backing Ms Truss in the campaign, said the post did not represent the foreign secretary’s views, adding: “It’s certainly not the sort of thing I would tweet.”

He told Sky News that Ms Dorries is “well-known for having strong views on things” and “speaks for herself”, but said: “I think we have all got to make sure that we remember we are one party, we come back together, we work together.

“We will have different views and the whole point of the leadership campaign is that both the candidates and their teams outline their differences of opinion so that the members can then choose who they think is the best person to lead our party and to lead our country.”

Meanwhile, the candidates are continuing to make their policy pitches to the Conservative Party membership, who will begin voting this week.

Mr Sunak has pledged to introduce a temporary £10 fine for NHS patients who fail to attend a GP appointment, while Ms Truss described herself as the “education prime minister” with a plan that includes replacing failing academies with “a new wave of free schools”.

The pair will campaign over the summer, with a number of hustings and a head-to-head debate on Sky News, hosted by Kay Burley.

The winner will then be announced on 5 September, and is expected to move into Number 10 the day after.

Rishi Sunak and Liz Truss will take part in a head-to-head debate on Sky News on Thursday 4 August at 8pm hosted by Kay Burley.

If you would like to be a member of the live studio audience and be in with a chance of asking a question, please apply here.