Scotland Yard today announced that it will launch an investigation into alleged cases of the misuse of expenses by MPs.

At 5.10pm, the Metropolitan police announced an inquiry into a "small number" of MPs and peers would be carried out, but refused to confirm names.

Police had originally said they would not be pursuing the matter.

The decision to launch an investigation came as the leaders of the major political parties announced that, in future, all expenses would be published in full with no redcation of key information.

The next tranche of MP's expenses, for the financial year that ended in April, is due to be published in October.

Officers from the Met's economic and specialist crime command will conduct the investigation, which will be overseen by the Met's temporary assistant commissioner, Janet Williams, a former special branch commander.

In a statement, the force said: "After consideration by the joint Metropolitan police and Crown Prosecution Service assessment panel, the Met has decided to launch an investigation into the alleged misuse of expenses by a small number of MPs and peers."

Police sources had previously indicated that, if any criminal investigations were launched, they would be into allegations of fraud.

Sources say those cases in which an MP might have claimed interest for a mortgage they had paid off might warrant a criminal inquiry for fraud. Other inquiries could be in to phantom travel expenses.

Labour MPs Elliot Morley and David Chaytor both continued to claim for mortgage interest payments despite having paid off their loans - £16,000 and £13,000 respectively. Labour peer Baroness Uddin is accused of claiming as much as £100,000 for a property reportedly left empty, while the Conservative Bill Wiggin claimed £11,000 in parliamentary allowances for a property he owned outright.

Police and leading prosecutors have spent several weeks going through suspicious claims from MPs and peers.

Officers have interviewed staff in the fees office as part of their inquiry to satisfy themselves that MPs were following the rules in most cases.

They are refusing to say how soon any charges if any could be brought.

Parliament today published a list of MPs who have repaid claims, suggesting that 182 from all parties had returned a total of £478,616.

Most MPs insist their behaviour was within the rules and that they paid money back to alleviate voter anger.