A Brexiteer Tory MP has been slapped down by a Foreign Office minister after suggesting the diplomat at the centre of a leak scandal was unfit for the job because he was too pro-EU.

Sir Bill Cash was branded “deeply unworthy” after claiming Sir Kim Darroch, the British ambassador to the US, lacked judgment because he was “prejudiced” about Brussels.

The veteran Brexiteer also said leaked memos written by Darroch, which described Donald Trump’s administration as “inept”, were “toxic” and “unjustified”.

But responding in the Commons, fellow Tory MP and Foreign Office minister Sir Alan Duncan defended the diplomat’s “reporting of the highest quality”.

Later, Duncan also joked that he threw something at the radio after hearing Nigel Farage criticising Darroch in an interview on Monday morning.

The government has launched an inquiry to find the identity of the leaker, with Foreign Secretary Jeremy Hunt warning the culprit faces “very serious consequences”.

During an urgent question in the Commons on the scandal, Cash faced shouts of “shame on you” as he questioned Darroch’s suitability for the job following the leaks to the Mail on Sunday.

The Eurosecptic MP said: “These toxic and unjustified attacks on the president of the United States and his administration are regarded by many people as completely unjustified.

“As chairman of the European scrutiny committee I was more than well aware of Darroch’s own prejudices in relation to the EU.

“Surely it is not his so-called frankness which should be the issue but his lack of judgment, which disqualifies him from his post.”

Duncan hit back: “I consider (your) intervention deeply unworthy.

“He is a diplomat of calibre, and of integrity, nothing in his reporting from the embassy could ever be construed as an attack on the president of the United States, all of it was reporting of the highest quality which we expect of our diplomats and our diplomatic network.”

Earlier Farage, a long-term critic of Darroch and an ally of Trump, said the ambassador’s confidential comments proved he was “totally unsuitable for the job and the sooner he is gone the better”.

But the Brexit Party leader was criticised on both sides of the Commons for his claims.

Labour’s Madeline Moon said: “During my journey to the station so many people I spoke to tuned into Radio 4 and the minute they brought in Nigel Farage to make his comment they immediately turned off because they felt his opinion on Sir Kim was so appalling.”

Duncan replied: “I also heard him on the radio and after throwing something at it I then switched it off.”

Commons foreign affairs committee chairman Tom Tugendhat earlier revealed he had written to Metropolitan Police Commissioner Cressida Dick “to ask that a criminal investigation also be opened into the leak”.

Duncan said if a leak culprit could be traced then they should “regret the moment for the rest of their life”, amid suggestions for security service involvement in the inquiry.

Conservative former minister Richard Benyon praised Darroch’s “balanced” and “professional” briefings, adding: “I also hope a message will go out right across the diplomatic service and to ministers – potential ministers as well – that all the agencies at the resource of the government will be used in inquiries and those found to have done this will really regret having done it.”

Duncan replied: “The Cabinet Office will use all of the means it can to delve into this and try and find a culprit.

“I wholly agree with (Benyon) that if we succeed in tracing who did this, they should regret the moment for the rest of their life.”