The standards watchdog suggested stripping the prime minister of power to rule on ministerial behaviour today, in the wake of the Priti Patel bullying row.
Lord Evans told MPs that allowing Boris Johnson or any other PM have the final say on whether Cabinet members should be investigated - and then decide if they had broken the rules - was like 'marking your own homework'.
It came after an official inquiry into Ms Patel, the Home Secretary, found that she had bullied civil servants.
But Mr Johnson refused to sack her because he deemed that her behaviour did not breach the ministerial code, as it also criticised her staff.
Lord Evans, chairman of the Committee on Standards in Public Life, suggested bringing the system more into line with that for MPs, in which they are investigated by an independent Parliamentary Commissioner for Standards.
They determines whether rules have been broken and it is then up to a crossbench committee of MPs to determine a punishment.
He said: 'The independent elements that exist in the Commons system and have been introduced into the Lords system now don't apply to the ministerial code.
'So I think there's a question to be asked about whether there should be more independence, whether the investigative element should be triggered independently, potentially, and then there's the separate but parallel issue of what the response to the actual investigation should be.'
He added: 'If bullying is treated in one particular way for MPs in their parliamentary role then why would you want to handle it differently in their ministerial role?'
Lord Evans told MPs that allowing Boris Johnson or any other PM have the final say on whether Cabinet members should be investigated - and then decide if they had broken the rules - was like 'marking your own homework'
It came after an official inquiry into Ms Patel, the Home Secretary, found that she had bullied civil servants
But Mr Johnson refused to sack her because he deemed that her behaviour did not breach the ministerial code, as it also criticised her staff
Ms Patel, 48, apologised for her treatment of civil servants on Friday after the official investigation found she bullied staff, on occasion shouting and swearing at them.
Johnson defied his critics to rally Tory MPs around her, disagreeing with his standards adviser and ruling her behaviour did not break conduct rules for ministers.
However, reports at the weekend suggested she could be demoted in a New Year reshuffle to becoem Conservative Party Chairwoman.
Kit Malthouse, the policing minister, who worked for Mr Johnson when he was mayor of London, was touted as a possible replacement.
The decision to keep her in her post - taken during the UK's anti-bullying week - sparked a furious new row in Westminster at a time when Mr Johnson is attempting to rest his government after the departure of top aide Dominic Cummings.
Her allies have defended her management style, with one telling the Sunday Times: 'People in real life do not carry on as if everything is like an Enid Blyton novel and sometimes things go wrong and people say things out of frustration that they would not say to their mother,'
Another lashed out at the 'racism and misogyny' she had suffered at the Home Office, claiming it was 'the worst of her career'.