Two of the key media partners in the Paradise Papers investigation have condemned the legal action that is seeking to force the Guardian and the BBC to disclose documents used by reporters.

Wolfgang Krach, the editor-in-chief of Süddeutsche Zeitung, the German paper that obtained the data, said he was “extremely worried about the attempt to force a journalistic enterprise to hand over highly sensitive data that could endanger the life and wellbeing of sources”.

Gerard Ryle, the director of the International Consortium of Investigative Journalists (ICIJ), which coordinated the Paradise Papers project, described the legal challenge as “a potentially dangerous moment for free expression in Britain”.

They reacted after Appleby, the offshore company at the heart of the story, launched breach-of-confidence proceedings against the Guardian and the BBC.

In legal correspondence, Appleby has also demanded that the Guardian and the BBC disclose any of the 6m Appleby documents that informed their reporting for a project that provoked worldwide anger and debate over the tax dodges used by individuals and multinational companies.

Appleby is also seeking damages for the disclosure of what it says are confidential legal documents. The Guardian and the BBC have said they intend to robustly defend the legal action.

The Paradise Papers project included 380 journalists from 96 media organisations across 67 countries. The consortium included the New York Times, Le Monde, the ABC in Australia and CBC News in Canada.

The project revealed details of the complex arrangements and offshore activities of some of the world’s richest people and companies.

It has already provoked a formal inquiry by the Australian tax office, a review by HMRC into VAT schemes on the Isle of Man, and calls from the EU finance commissioner, Pierre Moscovici, for changes in the law to stop “vampires” avoiding paying tax.

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. It has brought legal action against only the Guardian and the BBC, both UK-based media organisations.

Krach said: “Süddeutsche Zeitung, who initially received the Paradise Papers, would not allow the Guardian, or any other partner, to make the leaked documents available to third parties.

“At the same time Süddeutsche Zeitung is extremely worried about the attempt to force a journalistic enterprise to hand over highly sensitive data that could endanger the life and wellbeing of sources.

“Journalists must be allowed to protect their sources by all means especially when they clearly report in public interest. Therefore we appeal to the court and the public to support the Guardian’s legitimate wish to keep the material protected.”

Ryle said the Paradise Papers project had allowed “journalists across the world … to bring a new kind of scrutiny to power”.

“The BBC and the Guardian have been part of recent collaborations into financial secrecy that have changed laws from the US to New Zealand to Europe, sending a strong message to the corporate world that some of the behaviour we revealed is no longer acceptable.”

A spokesman for the Guardian said the claim could have profound consequences, and deter British media organisations from undertaking serious, investigative journalism in the public interest.

He 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 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.”

Appleby said it was “obliged to take legal action”.

It 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.”