The Commons Public Administration Committee said it was "inappropriate" to describe Sir Philip Mawer – who was appointed to the post of "independent adviser" by Mr Brown when he became Prime Minister last year – as having real independence.

It said Sir Philip had no power to launch his own investigations, no staff of his own, and could be dismissed at any time on the whim of the Prime Minister.

"The post of independent adviser meets none of the criteria we associate with independence," the committee said.

"The holder of the post, Sir Philip Mawer, has been appointed by the Prime Minister on a non-specific term of office which can be terminated by the Prime Minister at any time and on any grounds.

"He has no staff of his own, no office and no budget, but relies on the Cabinet Office for all these things. There has been no open advertisement process and no parliamentary involvement in the appointment."

The MPs added: "Until these defects are remedied, we have difficulty accepting the suggestion that the new investigator can meaningfully be considered to be independent."

In particular, the committee said that Sir Philip should not have to wait for a request from the Prime Minister before mounting an investigation into an alleged breach of the Ministerial Code of Conduct.

"It is hard to see how the independent adviser can command public confidence if the Prime Minister can decide that prima facie breaches of the code will not be investigated," he said.

"Put simply, there is no point in having an investigator in post if he is not given discretion to investigate very public allegations that the code has been breached."

The post of independent adviser was created by Tony Blair in March 2006 in the wake of the row over the business dealings of the husband of then Culture Secretary Tessa Jowell – the lawyer David Mills. The first holder of the post was the then head of the National Audit Office, Sir John Bourn.

But the committee disclosed that although it was agreed that he should be paid a salary of £30,000-a-year in the role, he was never actually asked to carry out any investigations.

While it acknowledged that the remit of the adviser had been widened under Mr Brown, the committee said that the changes still did not go far enough.

Committee chairman Tony Wright said: "It is in the Government's interest to get this right.

"Ministers should be innocent until proven guilty like the rest of us – but the public won't swallow that as long as the Prime Minister still acts as judge, jury and executioner."

The committee also suggested that there should be some form of "yellow card" sanction for Ministers who committed less serious breaches of the code, short of requiring them to resign.

"Expecting Ministers to come to the House and make a formal statement of apology is one option; simple naming and shaming is another," it said.