The House of Lords Standards Commissioner has launched a formal investigation into Cabinet minister Baroness Warsi's expenses claims.
Former police chief Paul Kernaghan decided to investigate after being asked to look into the claims by Lady Warsi.
She is facing allegations that she claimed for accommodation expenses while staying at a friend's house rent-free.
Scotland Yard said it had decided not to investigate Lady Warsi's expenses and had passed the matter back to the House of Lords.
Labour had called for a criminal inquiry into the Tory peer's expenses.