BT and Vodafone are among seven large telecoms firms which could be pulled into a legal challenge under human rights law for cooperating with GCHQ's large-scale internet surveillance programs.

Lawyers for the group Privacy International, whose mission is to defend the right to privacy, have written to the chief executives of the telecoms companies identified last week by the German paper Süddeutsche and the Guardian as collaborating in GCHQ's Tempora program.

Tempora is an internet buffer that lets analysts search vast databases of metadata on internet traffic crossing the UK, for up to 30 days after data is sent. Content of communications is retained for up to three days.

The lawyers' letter gives BT, Verizon Business, Vodafone Cable, Global Crossing, Level 3, Viatel and Interoute two weeks to answer questions about their policies on compliance with the intelligence-gathering program. They have also been asked about what legal requests they have received, and about any payments for giving intelligence agencies access for tapping internet cables.

If the firms refuse to comply Privacy International says they will be added as respondents to a legal claim to the Investigatory Powers Tribunal, which is challenging the Tempora program as a possible violation of the European convention on human rights.

The claim was filed by the group in July following the revelation of the program in the Guardian this June. The letter to the chief executives quotes a recent report from Frank La Rue, the UN special rapporteur on freedom of opinion and expression, which noted that "by placing taps on the fibre optic cables, through which the majority of digital communication information flows, and applying word, voice and speech recognition, states can achieve almost complete control of tele- and online communications".

He continued that in his view such mass interception "eradicates any considerations of proportionality, enabling indiscriminate surveillance".

GCHQ's internet monitoring allows it to tap up to a quarter of internet traffic flowing through the UK at any one time, then use the NSA-developed XKEYSCORE system to search for individuals' communications, search terms, browsing habits, and more. GCHQ and NSA analysts have direct access to the data collected.

When contacted last week, none of the telecoms companies gave any specific responses to questions about participation in the Tempora program, but several noted they were obliged to act in line with UK and European law, including responding to requests from government and law enforcement.

A Vodafone spokesman said : "Vodafone does not disclose any customer data in any jurisdiction unless legally required to do so. Questions related to national security are a matter for governments not telecommunications operators."

Eric King, head of research for Privacy International, said: "Despite the companies' obligation to respect human rights standards, particularly when governments seek to violate them, spy agencies are being allowed to conduct mass surveillance on their systems … human rights obligations are especially important when the government requests are made without public scrutiny, and are governed by secret law.

"These companies are the last line of defence for customers against the government's intrusion into our private lives. What we, and the public, deserve to know is this: to what extent are companies cooperating with disproportionate intelligence gathering, and are they doing anything to protect our right to privacy?"