Appleby, the offshore law firm at the centre of the data hack dubbed the 'Paradise Papers', has turned to Osborne Clarke for advice as it plans legal action against The Guardian and the BBC.

The offshore firm confirmed that it has instructed top 30 UK firm Osborne Clarke as it plans to launch breach of confidence proceedings against both news outlets. Commercial litigation partner Ashley Hurst is advising the firm.

Appleby's plans were leaked to German newspaper Suddeutsche Zeitung (SZ) and then passed on to the US-based International Consortium of Investigative Journalists (ICIJ).

According to reports in The Guardian, Appleby has also demanded that The Guardian and the BBC disclose any of the six million Appleby documents used to inform their reports following the hack. The newspaper has confirmed that it intends to defend the legal action.

In a statement, an Appleby spokesperson said: "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 BBC spokesperson said: "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."

A Guardian spokesperson added: "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."

Appleby is one of the world's best-known offshore law firms. Besides Bermuda, Appleby also has offices in the British Virgin Islands, Cayman Islands, Hong Kong, Mauritius, Seychelles and Shanghai, as well as the tax-friendly Crown Dependencies of Guernsey, the Isle of Man and Jersey.

Its major clients include Uber, Apple, Nike, Glencore and accounting giants KPMG, EY and PwC, according to the ICIJ.

At the time of the hack, the firm said in a statement that having "thoroughly and vigorously investigated the allegations" they were "satisfied that there is no evidence of any wrongdoing, either on the part of ourselves or our clients".

The ICIJ said the documents revealed "how deeply the offshore financial system is entangled with the overlapping worlds of political players, private wealth and corporate giants, […] that avoid taxes through increasingly imaginative bookkeeping manoeuvres".

Clifford Chance, Allen & Overy, Berwin Leighton Paisner and Clyde & Co were among a number of major law firms named in the leaked documents.