Rupert Murdoch was accused today of gagging the royal family's lawyers amid claims that key evidence in the phone hacking and bribery scandals has been held back from MPs.

In a sensational statement, legal firm Harbottle & Lewis said it wanted to speak out but had been forbidden by its client, News International, which owned the News of the World. Senior MPs said they may order News International to release the lawyers from their duty of client confidentiality, to ensure all relevant information is revealed.

Harbottle & Lewis has represented royalty including the Duke and Duchess of Cambridge, the Prince of Wales and the Duke of York. Its statement was released hours after Mr Murdoch and former chief prosecutor Lord Macdonald gave evidence to MPs. It said: "News International representatives referred to our advice in their statements ... before the parliamentary select committee, both as a result of questioning and on their own account.

"We asked News International to release us from our professional duties of confidentiality in order that we could respond to any inaccurate statements or contentions and to explain events in 2007. News International declined that request, and so we are still unable to respond in any detail as to our advice or the scope of our instructions in 2007, which is a matter of great regret."

The firm's role in the affair has been critical. In 2007 it took possession of hundreds of internal emails from News International which are now known to contain incriminating material. The file was finally handed to the police on June 20 this year.

In evidence to the Commons culture committee, James Murdoch said he believed assurances from Harbottle & Lewis that the emails did not show wider evidence of criminality. But Lord Macdonald, former director of public prosecutions, told the home affairs committee it took him less that five minutes to find evidence of serious offences. "It was impossible to look at that file not to see crime in its face," he said. He went to the News International board this year, which agreed the police must be told.

Keith Vaz, chairman of the home affairs committee, said: "I will write to Harbottle & Lewis to ask if they wish to clarify any of the statements made about them by Lord Macdonald, or if they have any new evidence to put before the committee" Labour MP Tom Watson, from the culture committee, said: "Harbottle & Lewis must be able to speak out. If News International holds them to client confidentiality they will suffer catastrophic reputational damage. We are inching closer to the truth and this will help us."

Fresh evidence from the lawyers could shed light on News International's failure to give police incriminating material until recently. The files were reviewed in 2007 by the company's legal director, John Chapman, and Daniel Cloke, former human resources director. John Yates, the assistant Met commissioner who resigned this week, told MPs: "the facts appear to be that News International have deliberately covered up" evidence.

Lord Macdonald said the file was put together in 2007 when Clive Goodman, former royal reporter at NoW, was bringing an unfair dismissal claim against News International. He said Harbottle & Lewis were "asked to give an opinion as to whether that information that had been gathered supported phone hacking".