London's mayor has been accused of political interference in the police handling of the phone-hacking scandal.
Labour MPs claim Boris Johnson was wrong to dismiss the allegations as "codswallop" when inquiries into the News of the World were active.
They have also demanded to know what his policing deputy, Kit Malthouse, knew about the probe when Mr Johnson made the remarks last September.
A spokesman for the mayor said the claims were "ridiculous".
Mr Johnson made his comments two weeks after the New York Times made claims that Andy Coulson - the then the prime minister's press chief - had known about and approved phone hacking at the News of the World.
Labour members of the assembly have said the mayor should not have "intervened" during the course of an active police inquiry.
Shadow home secretary Yvette Cooper said: "It does make it harder for the Met to do their job if the mayor is saying that some of the allegations they are supposed to be investigating are codswallop or are politically motivated.
"In fact we wanted the Met to open this investigation at a much earlier stage and it is questionable whether that was harder for them because of what the mayor did."
Labour also wants Lord Justice Leveson's public inquiry to examine the mayor's role in the hacking scandal.
Mr Johnson's spokesman said: "The mayor did not put any pressure on the police not to reopen their inquiry.
"The mayor has already encouraged Lord Leveson to look carefully at relationships within and around the Met, and in fact was one of the first to do so, and would naturally expect to be given a chance to share his reflections during this process."
'Open and honest'
The role of Mr Malthouse has also fallen under scrutiny.
Members of the Metropolitan Police Authority have written to him asking what he knew when the mayor made his remarks.
They have also questioned why he failed to tell them last week that Neil Wallis - former deputy editor of the News of the World - had been hired by the Metropolitan Police.
Jenny Jones, Green Party member of the London Assembly, said: "I think he's definitely got to answer questions from the authority and he has to answer us fully and openly and honestly and I think so far that's something we haven't had from him."