London Mayor Boris Johnson has asked the commissioner of the Metropolitan Police, Sir Paul Stephenson, to explain why former-News of the World executive Neil Wallis, who was arrested Thursday in relation to the phone hacking scandal, worked as a paid consultant for Scotland Yard.

Wallis was on the Met's payroll for almost two years until September 2010 as sick leave cover for the force's deputy director of public affairs, it was revealed Thursday. During this period Assistant Commissioner John Yates decided there was no need to reopen the inquiry into phone hacking.

Willis earned £24,000 for his work for the Met for two days a month of work. The Met claim Willis' company simply offered the lowest price for the work.

Johnson called Stephenson to City Hall and told him to investigate the claims as park of the phone hacking inquiry in what has been described as a dramatic and "very frank" discussion. Home Secretary Theresa May has written to Sir Paul to get "the full picture" of what happened, and Stephenson has also agreed to give evidence to the Home Affairs Committee on Tuesday at the request of committee chair Keith Vaz.

The Met said: "Chamy Media, owned by Neil Wallis, former executive editor of the News of the World, was appointed to provide strategic communication advice and support to the MPS, including advice on speechwriting and PR activity, while the Met's deputy director of public affairs was on extended sick leave recovering from a serious illness."

"In line with Metropolitan Police Service/Metropolitan Police Authority procurement procedures, three relevant companies were invited to provide costings for this service on the basis of two days per month. Chamy Media were appointed as they were significantly cheaper than the others. The contract ran from October 2009 until September 2010, when it was terminated by mutual consent."

At a meeting of the Metropolitan Police Authority earlier on Thursday, Stephenson was asked to defend his decision to dine with Wallis in 2006 when he was still working for the News of the World.

"I do not believe that on any occasion I have acted inappropriately. I am very satisfied with my own integrity," he said.