News Corporation's defence that phone hacking at the News of the World was the work of a single "rogue reporter" was on the verge of collapse tonight after Glenn Mulcaire, the private detective at the centre of the case, said the paper's head of news commissioned him to access voicemail messages.

Mulcaire is understood to have submitted a statement to the high court this afternoon confirming that Ian Edmondson, the paper's assistant editor (news) asked him to hack into voicemail messages left on a mobile phone belonging to Sky Andrew, a football agent. Andrew is suing the paper for breach of privacy.

It is also understood that Mulcaire said in the court statement that several other executives at the News of the World were aware that phone hacking was taking place, although he does not name them.

A spokesman for the News of the World said: "This is a serious allegation that will form part of our internal investigation."

Edmondson was suspended by the paper before Christmas after he was named in court documents in a separate case against the News of the World brought by the actor Sienna Miller.

His computer has been impounded as part of the paper's internal investigation and the company is trawling through his emails. He is expected to be questioned after colleagues have been interviewed.

Mulcaire's decision to name Edmondson helps to explains why News Group acted so quickly to suspend him.

Mulcaire's lawyer, Sarah Webb, said: "It's in court documents. I'm not prepared to comment."

The admission by Mulcaire, whose legal fees are believed to be met by News of the World publisher, News Group, which is part of Rupert Murdoch's media empire, contradicts the paper's repeated claim that only a single journalist – the former royal editor Clive Goodman – knew about his activities. Executives at the paper, including its former editor Andy Coulson, now David Cameron's director of communications – have stuck to that version of events since Goodman and Mulcaire was jailed in 2007 for illegally intercepting voicemails left on mobile phones belonging to members of the royal household.

A Downing Street spokeswoman said: "We have nothing further to add."

Files seized by police in a 2006 raid on Mulcaire's home show that Mulcaire wrote "Ian" in the margins of a transcript he made of messages left on Miller's phone.

Miller's lawyers had contended that "Ian" referred to Edmondson, an executive at the paper who was hired by Coulson and worked closely with the former editor during his time at the paper.

Mulcaire had a habit of writing the first name of whoever had asked him to conduct hacking in the top left corner of his paperwork. His conviction in 2006 along with Goodman rested partly on the fact he had written "Clive" on his files.

Lawyers acting for Nicola Phillips, a publicist suing the paper for breach of privacy, won a high court ruling in November ordering Mulcaire to name the executives who ordered him to hack into phones.

He appealed against that ruling, however, on the grounds that he could incriminate himself by doing so, and the court of appeal has yet to hear his case.

It is unclear why Mulcaire has decided to name Edmondson now, although it is thought lawyers acting for several other litigants, including the comedian Steve Coogan and the Sky Sports commentator Andy Gray are preparing to make the same request. Murdoch has pledged "immediate action" against anyone found hacking again. News Corporation had fought a long battle to prevent details of the phone-hacking affair becoming public.

The Guardian revealed in July 2009 that News Corp had paid the PFA chief executive, Gordon Taylor, and two others a total of £1m in a secret out-of -court settlement in exchange for dropping a hacking case. The documents relating to the case were then sealed by the court. The celebrity publicist Max Clifford received £1m last year in a similar settlement.