Bermudian-founded law firm Appleby has launched breach of confidence proceedings against the Guardian and the BBC.

According to the Guardian, Appleby has also demanded that the two news organisations disclose any of the six million documents that informed their reporting of the Paradise Papers investigation.

The revelations, which pitched Bermuda in the middle of an international storm over tax affairs, were unveiled after a year’s work by the International Consortium of Investigative Journalists. Files include financial statements, e-mails and loan arrangements going back decades.

The Guardian and BBC say they will “defend robustly” the legal action.

Appleby has said the documents were stolen in a cyber-hack and there was no public interest in the stories published about it and its clients.

Appleby said in a statement on the Guardian: “Our overwhelming responsibility is to our clients and our own colleagues who have had their private and confidential information taken in what was a criminal act. We need to know firstly which of their — and our — documents were taken.

“We would want to explain in detail to our clients and our colleagues the extent to which their confidentiality has been attacked. Despite repeated requests the journalists have failed to provide to us copies of the stolen documents they claim to have seen. For this reason, Appleby is obliged to take legal action in order to ascertain what information has been stolen.”

A spokesman for the Guardian said: “We can confirm that a claim has been issued against the Guardian. The claim does not challenge the truth of the stories we published. Instead it is an attempt to undermine our responsible public interest journalism and to force us to to disclose documents that we regard as journalistic material.

“This claim could have serious consequences for investigative journalism in the UK. Ninety-six of the world’s most respected media organisations concluded there was significant public interest in undertaking the Paradise Papers project and hundreds of articles have been published in recent weeks as a result of the work undertaken by partners. We will be defending ourselves vigorously against this claim as we believe our reporting was responsible and a matter of legitimate public interest.”

A BBC spokesman stated in the Guardian article: “The BBC will strongly defend its role and conduct in the Paradise Papers project.

“Our serious and responsible journalism is resulting in revelations which are clearly of the highest public interest and has revealed matters which would otherwise have remained secret. Already we are seeing authorities taking action as a consequence.”