It has been reported that the politician is Labour peer Baroness Uddin, who is alleged to have wrongly claimed £100,000 of expenses.

The CPS is due to reveal their decision in the morning.

It comes after three Labour MPs and a Tory peer pleaded not guilty to charges relating to false accounting over their expense claims.

Elliot Morley, David Chaytor, Jim Devine and Lord Hanningfield denied any wrongdoing when they appeared in the dock at the City of Westminster Magistrates' Court.

The men will next appear before a Crown Court on March 30, on charges of false accounting.

District Judge Timothy Workman declined jurisdiction on the case, claiming it raised issues of constitutional importance and should be tested at a higher court.

Julian Knowles, the MPs' barrister, submitted an application for them to sit outside the dock during the hearing, but it was rejected.

If found guilty of abusing the Parliamentary allowance system, they face being jailed for up to seven years.

Morley, MP for Scunthorpe, is accused of dishonestly claiming over £30,000 more than he was entitled to on a house in Winterton, including an 18-month period after the mortgage had been paid off.

Chaytor is charged with false expenses claims of £20,375 and Devine, who represents Livingston, is alleged to have been paid £9,000 for cleaning services and stationary between 2008 and 2009.

Lord Hanningfield, whose appearance followed the MPs, faces six counts of false accounting.

The charges relate to claims for overnight allowances, when he was allegedly driven home to Chelmsford.

Meanwhile, police have begun an enquiry into a fifth Labour MP, Harry Cohen, over his expenses.

Last month the Standards and Privileges Committee concluded Cohen 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 CPS announcement does not relate to Mr Cohen because the Metropolitan Police have yet to hand over his file to the CPS.