Lawyers acting for a prominent Libyan dissident who was tortured after he was seized in a joint British and US operation have said they believe their communications are being intercepted by GCHQ, the government's electronic eavesdropping agency.

They are seeking an urgent injunction from the investigatory powers tribunal, which hears complaints about the activities of the security and intelligence agencies. They lodged a complaint after revelations about GCHQ's mass surveillance programmes, leaked by Edward Snowden, were published in the Guardian.

Their suspicions were further aroused when, after a long delay, lawyers acting for Britain's intelligence agencies demanded that part of the tribunal hearings should be held behind closed doors, without the presence of lawyers representing the Libyan, Abdel Hakim Belhaj.

Lawyers acting for GCHQ and other UK agencies told the tribunal that after "undertaking searches" they had come to the conclusion that part of the hearings should be closed.

In response, Leigh Day, lawyers for Belhaj and his family, said it was "obvious that our clients have been subject to interception, and that interception has included their privileged communications".

The government's lawyers have declined to give written assurances that communications between Belhaj and his lawyers have not been intercepted.

Speaking from Tripoli, Belhaj said on Wednesday: "When I was in Gaddafi's prisons I used to find all kinds of bugging devices in my cell, and I expected it. But I find it surprising that the UK government insists on listening in when my wife and I talk to our lawyers about our torture case. Both of us still have faith in the British justice system, but that doesn't sound like fair play from the UK government to us."

Cori Crider of the legal charity Reprieve, which is also acting for Belhaj, said: "Anybody who still imagines the Guardian's mass spying revelations are just about our Facebook selfies needs to take a hard look at this case. First MI6 (with the CIA) kidnaps Belhaj and his pregnant wife and sends them to Gaddafi's torture chambers in 2004. Then the government refuses the family's offer to settle for £3 and an apology. Now it won't even promise to let the couple talk to us, their lawyers, without listening in."

Belhaj is suing Britain's intelligence agencies and Jack Straw, who was foreign secretary at the time of the rendition operation, in the British courts. The government argues that courts here cannot hear the case since any rendition operation and flights in 2004 took place abroad, not in the UK.