The Crown Prosecution Service, which has been studying the evidence against six parliamentarians, will announce its decision at 11am.
Labour MPs Eliot Morley, Jim Devine and David Chaytor have been under investigation by Scotland Yard for claiming for mortgages that had already been paid off. All three deny wrongdoing.
Labour peer Baroness Uddin is also known to be under investigation, along with two Tory peers, Lord Clarke of Hampstead and Lord Hanningfield.
If the CPS does decide to prosecute, it is likely to bring charges of fraud or false accounting.
The news comes hot on the heels of Sir Thomas Legg’s report into MPs’ expenses, which was published yesterday. It found that 390 MPs had broken the rules. They were ordered to repay a total of £1.3m. But Sir Paul Kennedy, a former judge who was asked to consider appeals against Legg’s rulings, revealed that 44 out of 75 appeals had been successful.