The government has announced the honours system is to be overhauled.

The Cabinet Office confirmed on Monday that work was "in hand to review the structure and membership of the honours committees".

Departmental permanent secretary Sir Hayden Philips is expected to conduct a thorough review with the aim of encouraging transparency in the secretive nominations process.

The move follows a series of embarrassing leaks and criticisms of the system.

It has been revealed that several celebrities have rejected knighthoods, OBEs and MBEs because of the awards' associations with the British Empire.

Meanwhile science minister Lord Sainsbury was forced to apologise for a leaked honours snub to scientist Professor Colin Blakemore.

The head of the Medical Research Council was denied an award in the new year's honours list because of his controversial defence of experiments on animals.

Professor Blakemore had threatened to resign from the official post he recently assumed unless more backing was given to him by ministers.

"It has nothing to do with whether I particularly deserve an honour - that is neither here nor there," he told the BBC.

"The mission statement of the medical research organisation which I now run includes a specific commitment to engaging with the public on issues in medical research.

"How can I now, in the present circumstances, go to MRC scientists and ask them to take the risk of being willing to talk about animal experimentation with this indication that doing so will reduce their standing and their reputation in the eyes of the government?"

Lord Sainsbury said the leak sent out the "wrong message".

"I want at this point to say on behalf of this government this does not in any way represent government policy," he said.

"This is essentially a civil service process and it does not represent government policy, which is quite clear on this.

"It is well known I think that government believes it is necessary to do animal experiments within the tough regulatory regime we have.

"It also, I think, is quite clear the government both admires and fully supports those on the frontline who have stood up to animal rights extremists."

But Liberal Democrat MP Dr Evan Harris called for an inquiry into why the Whitehall Honours Committee thought recognising the scientist's work would be unacceptable to ministers.

The Oxford West and Abingdon MP, who represents Professor Blakemore, said the government could not separate themselves from civil service decisions.

"Following the revelation that senior Whitehall mandarins blocked an honour for Professor Blakemore, it is not enough for the government to wheel out Lord Sainsbury, the science minister, to blame the civil service," said Harris, a member of the Commons science and technology committee.

"Such Whitehall committees do not work in isolation from their political masters; it is the fault of ministers that these civil servants were so clearly of the view that due recognition for Professor Blakemore and his colleagues was politically unacceptable and it is now down to ministers to correct that view publicly.

"Politicians of all political parties - including my own - have a poor record on this issue.

"If you want safe and effective medicines, you have to accept regulated research using animals, and you must defend utterly the right of such scientists to do that work free from fear and attack."