The speaker of Britain’s House of Commons Michael Martin will make a statement today on the arrest of Conservative immigration spokesman Damian Green.
Some MPs are threatening to stage a protest if he refuses to allow an emergency debate.
Many politicians are furious that Mr Martin apparently failed to uphold ancient parliament privileges by letting the police search Mr Green's Commons office last week.
Some have called for him to be sacked and have vowed to interrupt his statement if he rules out a debate.
The row threatens to overshadow the Queen's reopening of parliament this afternoon.
Mr Green (52), was questioned for nine hours by counter-terrorism officers after being arrested as part of an inquiry into Home Office leaks. He was detained on suspicion of "conspiring to commit misconduct in a public office and aiding and abetting, counselling or procuring misconduct in a public office".
Police said they acted after receiving a complaint from the Cabinet Office, the department in charge of the rules governing the conduct of government employees and ministers.
Paul Stephenson, acting commissioner of the Metropolitan Police, said officers had received written permission for the raid from the Sergeant-at-Arms, Jill Pay, who is responsible for the security of the House of Commons.
"Our priority is to conduct a thorough and lawful investigation," he told local politicians at London's City Hall. "It is our duty to follow the evidence wherever that may take us," he said.
Earlier, Conservative work and pensions spokesman Chris Grayling said Mr Martin needed to explain why the police raid was allowed. "The issue the speaker needs to address today is precisely what the status of parliamentary offices and the parliamentary estate is," he said.
"In the same way that police can't go into a judge's chambers and take sensitive legal documents in the middle of a case, I'm not convinced that police should be able to go into a parliamentary office and take away sensitive constituency papers."
But Business Secretary Lord Mandelson said the protests were a "self-serving smokescreen" for the Conservatives to hide their apparent collusion with a Home Office official who has been arrested on suspicion of leaking internal documents.
"The Civil Service operates on the basis of a code which seeks to uphold their impartiality and their professionalism," Mr Mandelson said.
"I would like to know from the Conservative Party whether their front bench and their leader knowingly colluded with that civil servant in riding a coach and horses, not only through the civil service code but also through the law."
Home Secretary Jacqui Smith, who is responsible for policing, has insisted she knew nothing about the impending action against Mr Green until detectives had carried out the raid.
In response to the row, the Metropolitan Police has asked the Chief Constable of the British Transport Police, Ian Johnston, to carry out an urgent review of its handling of its investigation.
Reuters