Jack Straw's unpaid "eyes and ears" in the Commons has resigned rather than embarrass the Home Secretary over an investigation into General Election expenses.

Colin Pickthall has stood down as Mr Straw's Parliamentary Private Secretary because of a police investigation into alleged discrepancies in his finances in the run-up to the 1997 poll.

A file has been passed from the Lancashire Police to the Crown Prosecution Service about the election expenses involving the 56-year-old West Lancashire MP and three other local Labour Party members.

Mr Pickthall is now waiting to find out if he will be prosecuted but has decided that he should stand down to save Mr Straw, responsible for law and order and the oversight of elections, from any embarrassment. The married former lecturer and ex-Lancashire county councillor worked closely with Blackburn MP Mr Straw liaising with back bench MPs in the Commons and generally running errands for him and reporting on the feeling in Parliament about his policies.

Mr Pickthall said: "I put my resignation to Jack Straw, which he has accepted with great reluctance. Our relationship remains very good. I am very sorry to do it, but there is no alternative." Mr Straw said: "I'm very sorry that Colin has gone. He's done an extremely good job for me and the Parliamentary Labour Party, however I understand his reasons."

Mr Pickthall was first elected to Parliament for West Lancashire, which includes Skelmersdale, in 1992. His majority at the last election was 17,119.