The head of Scotland Yard's probe into the "cash-for-honours" affair will be questioned by MPs as to why so long was spent investigating claims which did not lead to charges.
Metropolitan Police Assistant Commissioner John Yates will appear before the House of Commons Public Administration Select Committee on Tuesday as part of its inquiry into propriety in the honours system.
The committee's inquiry was put on hold last year at the request of Mr Yates while the Met investigated allegations that peerages were offered in return for financial support for Labour and the Conservatives ahead of the 2005 General Election.
But, after a police investigation lasting more than a year, the Crown Prosecution Service announced in July that no charges would be brought in relation to the Labour Party.
It emerged last week that there would be no prosecutions relating to Tory lenders, either.
A spokeswoman for the committee has said it is not interested in re-running the police inquiry, but in finding out what lessons could be learnt in terms of possible changes to legislation.
At a hearing last week, several members of the committee expressed frustration at the difficulty of bringing prosecutions under the "flawed" 1925 Honours (Prevention of Abuses) Act, which requires proof of an explicit agreement to sell honours.
Mr Yates will face questions about the conduct of the inquiry and why no charges were brought.
He will be joined before the committee by Carmen Dowd, the head of the Crown Prosecution Service's special crime division, which took the ultimate decision on whether to prosecute.
