The Law Society is considering issuing new guidance to solicitors across England and Wales amid growing concern that the government's mass online surveillance operations are undermining their ability to take legal cases against the state.

Lawyers representing people who make serious complaints against the police, army or security services fear the industrial-scale collection of email and phone messages revealed by the Guardian over the past four months is threatening the confidential relationship between them and their clients, jeopardising a crucial plank of the criminal justice system.

"These are absolutely fundamental issues," said Shamik Dutta, from Bhatt Murphy lawyers in London. "The NSA revelations are having a chilling effect on the way a crucial part of the justice system operates. Individuals who are making serious allegations of wrongdoing against the state are becoming increasingly concerned about whether the information they share with their lawyers will remain confidential."

Files leaked by the whistleblower Edward Snowden show the British eavesdropping centre, GCHQ, and its American counterpart, the National Security Agency, have developed capabilities to undertake mass surveillance of the web and mobile phone networks. This is done by trawling the servers of internet companies and collecting raw data from the undersea cables that carry web traffic.

Two of the programmes, Prism and Tempora, can sweep up vast amounts of private data, which is shared between the two countries. The Guardian recently revealed how GCHQ and the NSA have also successfully cracked much of the online encryption relied upon by hundreds of millions of businesses and individuals to protect their privacy.

The Law Society expressed concern about the possible impact on lawyers' day-to-day work. "The ability of clients to consult and receive advice from lawyers with certainty of absolute confidentiality is fundamental to the rule of law and the values of our democracy," it said in a statement. "The Law Society would regard it as very serious indeed if privileged correspondence were to be intercepted. We are looking at the issue and considering whether there needs to be advice given to the profession."

A spokesperson for the Bar Council said the "enormous developments in the cyber world over the past decade" meant it was right to question whether the proper safeguards were in place. "The public need to be reassured that powers to intercept communications are properly exercised. We have long had concerns that the existing legislation fails to respect the right of individuals to consult privately with a lawyer. Any increase in the scope of the authorities' data gathering activities must be matched by the establishment of real protection for this fundamental aspect of the administration of justice."

Eric King, head of research at Privacy International, said there were real fears in the legal profession about confidentiality being breached by the security services following the NSA revelations. "We are astonishingly concerned about privileged communications being swept up as part of the mass surveillance programmes we have learned about over the past few months," he said.

"When you are talking about litigation involving major cases that challenge significant government policy, the intelligence that could be gained and the change in strategy that could be deployed by obtaining an idea of what the claimants were seeking to challenge, it could easily tip the balance decisively in what are critical cases."

Several law firms are reviewing the way they communicate with clients, especially in sensitive cases against the government.

Clive Stafford Smith, director at Reprieve, who has acted in a number of high profile human rights cases, said: "What we have learned about the scale and reach of the security services' ability to monitor online communications over the past few months has far-reaching consequences for our ability to hold the state to account. I have periodically been told by people who would know that both the US and the UK snoop on my legal communications, and the impact of the latest revelations is obviously immense.

"A prisoner needs to be able to communicate confidentially with a lawyer, but this is impossible if he thinks that a legal discussion is just another form of interrogation. For example, my Guantánamo client Shaker Aamer hates to talk about his torture, as he thinks the people who are illegitimately listening in will just use his words to abuse him more effectively in future."

Dutta said Bhatt Murphy was one of the firms reviewing its online security. "Thankfully, we have effective ways of maintaining confidentiality. Nevertheless, it is deeply troubling that the victims of state misconduct should have these concerns; they should be able to rely upon the sanctity of solicitor-client privilege."

He said mass state surveillance had combined with the introduction of closed material proceedings in claims against the state and cuts to legal aid to drastically weaken citizens' abilities to hold the authorities to account. "This government is undermining access to justice on a number of fronts. and these new revelations are likely to further erode people's faith in the justice system."

Professor Ross Anderson, a cybersecurity expert at Cambridge University, told the Guardian the Snowden revelations were causing a crisis in the IT industry as lawyers, academics, doctors and engineers realise they might be snooped on. "How will the Prism affair affect ordinary middle-class people in Britain?" he said. "Surely we're of no interest to the analysts at the NSA? Yet some of our patients and clients surely will be."

With so much personal information being stored in 'cloud' based services, which are accessible to intelligence agencies, all professionals will have to rethink how they work, he said.

"It's time for the British Medical Association, Law Society, Bar Council and other professional bodies to start thinking about the ethics of using cloud-based services for confidential client information."