London Mayor Boris Johnson has shown "dire judgement" over the News of the World phone hacking scandal, Labour rival Ken Livingstone has said.
Mr Johnson originally dismissed hacking claims as "codswallop cooked up by the Labour Party" and should have pressed police harder, Mr Livingstone said.
City Hall said Mr Johnson had always stressed any "significant new facts" would need to be investigated.
News International said this Sunday's News of the World will be the last.
Mr Livingstone claimed the mayor "had at least two meals with Rebekah Brooks, one dinner and one lunch with James Murdoch, and one dinner with Rupert Murdoch" at a time when he was "trying to keep the lid on this story".
"Rather than speak truth to power, he defended the powerful and accountable," he added.
'Thousands of names'
But a spokesman for Mr Johnson said: "Senior police officers assured the mayor last September that the initial inquiry into these allegations had been thorough and conclusive.
"Since then new and far more serious allegations have come to light that no-one could have predicted, which have raised fresh questions that now need to be answered.
"The mayor said at the time that if significant new facts were brought into the public domain they would need further investigation, his position on which he has made fervently clear today."
The Metropolitan Police said it was sifting through about 11,000 pages of evidence, containing almost 4,000 names, in its investigation into alleged phone hacking involving the News of the World.
And the Independent Police Complaints Commission has been asked to supervise Scotland Yard's internal investigation into payments by journalists to police for information.
The paper's owner, News International, is co-operating with the police inquiry and is conducting its own investigation. On Thursday it announced it was closing down the paper for good.
The company said if the claims of hacking of military personnel's families were true, it would be "absolutely appalled and horrified".