Secret government policies which set out when lawyers’ or journalists’ phones and emails can be intercepted should be published, a court has been told.

In an open hearing of the investigatory powers tribunal (IPT), which examines complaints against the intelligence services and government use of surveillance, lawyers for two Libyan victims of rendition have called for the documents to be released.

The government’s refusal to reveal the policy papers has emerged as a key issue in the claim brought on behalf of Abdel Hakim Belhaj and Sami al-Saadi who, along with members of their families, were kidnapped and sent to face punishment in Libya in 2004.

The case before the IPT alleges that the intelligence agencies or government spied on their communications with their lawyers, damaging their right to a fair trial in their claim for compensation for kidnap and torture.

Communication between lawyers and their clients are deemed to be “privileged” under longstanding rules. Similar protection applies to the communications between journalists and their sources and other protected groups.

In a hearing at the IPT, Dinah Rose QC, representing the Libyans, said: “We don’t understand why it’s being said that disclosure of policy will cause harm to national security. None of this information ought to be secret. Procedures for ensuring that privileged material is properly protected ought to be open to public scrutiny.”

The government has declined to disclose policies regulating the circumstances in which these communications are intercepted and any safeguards in place to avoid abuse. It says they are secret.

At Thursday evening’s hearing, lawyers for the government did not explain why the policies could not be released. They did, however, hand over a summary of some of the policies in contention. It stated that as well as the existing Covert Surveillance and Interefence of Property Code of Practice, each of the intelligence agencies - the Security Service (MI5), Secret Intelligence Service (MI6) and GCHQ - had developed their own procedures.

The summary noted that: “The Security Service policy provides that no [legally privileged material] can be acted upon unless approval has been sought from a senior manager with advice from a Security Service lawyer.”

It concluded: “In terms of their future conduct, the agencies can confirm that if, not withstanding the [existing] safeguards, circumstances were to arise whereby a legal adviser or policy official with conduct of litigation (to which the relevant agency was a party) was to come into possession of [legally privileged material] belonging to another party, the agencies would seek advice from independent counsel.”

Further preliminary hearings will be held before the case is tried in November. One issue is whether the tribunal has the power to order the government to disclose documents, a principle that could turn into a major confrontation between civil rights groups and the government.

The IPT complaint is one of a series of cases after revelations by the CIA whistleblower Edward Snowden about monitoring of the internet and telephone calls by Britain’s eavesdropping agency, GCHQ, through its Tempora programme.

Eight Libyans, members of the two families, say they were victims of rendition. They claim they were kidnapped by MI6 and US intelligence agencies, forcibly returned to Muammar Gaddafi’s regime and tortured. At that time, in 2004, when Gaddafi relinquished his nuclear weapons programme, intelligence relations between Tripoli, London and Washington were close.

A separate legal action between Belhaj and the UK government is due to be heard at the high court to resolve compensation for the kidnap and torture allegations. The human rights group Reprieve, which is supporting the claim, fears its ability to fight the case will be undermined because staff’s legal correspondence may be surreptitiously monitored.

Saadi, another Libyan dissident, and his family have settled their claim against the government for a payment of £2.2m. The Foreign Office did not, however, admit liability.

The “notice of complaint” by solicitors at Leigh Day on behalf of Reprieve and the Libyans lists the Security Service (MI5), the Secret Intelligence Service (MI6), the Government Communications Headquarters (GCHQ) in Cheltenham, the home secretary and the foreign secretary as respondents. It calls for the case to be heard in open court. Most of the IPT’s hearings are in secret.

The claims states: “There is a strong likelihood that the respondents have intercepted and are intercepting the applicants’ legally privileged communications in respect of the [cases].”

Belhaj and Saadi were prominent military leaders of the Libyan Islamic Fighting Group during the revolution, the document points out, and are, therefore, “likely to be of interest” to UK intelligence agencies.