It is believed the person concerned is Labour peer Baroness Uddin, who is alleged to have wrongly claimed £100,000 worth of expenses.

It comes after three Labour MPs and a Conservative peer told a judge they will use a 320-year-old law to argue they should not be prosecuted.

MPs David Chaytor, Elliot Morley and Jim Devine, along with Lord Hanningfield, will insist their case should be dealt with by House of Commons authorities, not a jury.

In an unprecedented hearing at City of Westminster Magistrates' Court, all four said they would plead not guilty to false accounting charges.

District Judge Timothy Workman released them on unconditional bail to appear at Southwark Crown Court on March 30.

If convicted, they face a maximum sentence of seven years in prison.

Barrister Julian Knowles, for the MPs, told the court they would argue they were protected by parliamentary privilege, covered in the 1689 Bill of Rights.

"My clients should not be understood as saying that they are above the law - that would be quite wrong," he said.

"Parliamentary privilege is part of the law - and it is for Parliament to apply the law in their cases."

Bury North MP Chaytor, 60, of Todmorden, Lancashire, is accused of falsely claiming rent on a London flat he owned, falsely filing invoices for IT work and renting a property from his mother, against regulations.

Scunthorpe MP Morley, 57, of Winterton, North Lincolnshire, allegedly falsely claimed £30,428 in interest payments between 2004 and 2007 towards a mortgage on his home he had already paid off.

Livingston MP Devine, 56, of Bathgate, West Lothian, is said to have wrongly submitted two invoices worth a total of £5,505 for services provided by Armstrong Printing Limited.

He also faces a second charge alleging he dishonestly claimed cleaning and maintenance costs of £3,240.

Lord Hanningfield, 69, of West Hanningfield, near Chelmsford, Essex, faced six charges of making dishonest claims for travelling allowances.

Meanwhile, police have begun an inquiry into another Labour MP, Harry Cohen, over his expenses.

Last month the Standards and Privileges Committee concluded he had committed a "serious breach of the rules", by claiming more than £70,000 for a "second home" while renting out his main property.

Sky News understands the announcement due from the CPS does not relate to Mr Cohen because Scotland Yard has yet to hand over his file to prosecutors.