Embattled deputy prime minister Dominic Raab is under growing pressure amid calls for a third probe into his alleged behaviour.
The justice secretary faces fresh accusations that he used “obscene” language in the chamber of the House of Commons.
The row centres on claims that Mr Raab mouthed the word “w***er” at the Labour leader during Prime Minister’s Questions (PMQs) 10 days ago.
The apparent insult came as Sir Keir Starmer claimed the Conservative Party was “too weak to stand up for working people”.
Sir Alistair Graham, a former chairman of the Committee on Standards in Public Life, said the allegations should be investigated, with possible sanctions attached if they were found to be true.
A member of the Commons sleaze watchdog also hit out at the use of such language, warning that too many MPs were wrongly being given a “clean bill of health”.
The deputy prime minister is already being investigated after two formal complaints about his alleged behaviour were made earlier this week, with claims emerging that some staff were “scared” to go into his office while they worked for him.
One focuses on his time at the Foreign Office and the other on his first stint at the Ministry of Justice, where he was reinstalled as secretary of state by Rishi Sunak last month.
Mr Raab has denied all allegations of bullying and told MPs on Wednesday that he had “behaved professionally” throughout his career.
Asked if the accusations should be investigated by the Commons Speaker, even if he missed the alleged remark when it was originally said, Sir Alistair said “yes”.
He said that Speaker Lindsay Hoyle could say to members: “If you are aware of any member who is acting like this please draw it to my attention.”
Commenting on the alleged remark, he added: “Someone can be excluded [from the House of Commons] unless they apologise immediately. There have been plenty of MPs who have been subject to such disciplinary action by the Speaker.”
Rita Dexter, a lay member of the Commons Committee on Standards and a former deputy commissioner of the London Fire Brigade, complained that during “PMQs last Wednesday it was very clear that a frontbench member was mouthing something significantly obscene at the opposite bench members”.
At a meeting of the committee this week she criticised the fact the alleged behaviour “is not within the scope of any investigation” by the Commons standards commissioner.
“So it is not the case I think that there is a clean bill of health for 97 per cent [of MPs]. I think there are problems about that [calculation],” she said.
A source close to the committee said Ms Dexter was referring to the allegations against Mr Raab.
Another member of the standards committee said: “We are all very surprised that no action has been taken.”
Ms Dexter also complained that the committee had previously been told there could be no investigation into an alleged “obscene gesture” by an MP, despite 800 complaints, “because it was outside… scope”.
It is understood that Mr Raab categorically denies he said the word w***er and instead said “weaker” after Sir Keir used the word “weak” multiple times.
It is not the first time Mr Raab has been accused of mouthing the word w***er.
In 2021 he faced similar accusations after a clash with BBC Radio 4’s Nick Robinson.
At the time it was reported that both Mr Raab and the BBC declined to comment.
On Saturday, The Guardian reported allegations that Mr Raab’s refusal to speak to Foreign Office staff he considered “time-wasters” led to delays during the evacuation of Afghanistan.