Britain's eavesdropping agency GCHQ had extensive access to the records of telecom firms including BT and Vodafone, it has been reported.

The telecom giants have allegedly been passing on details of customers' phone calls and email conversations to the spy agency, according to the latest set of documents exposed by whistleblower Edward Snowden.

The firms, which include BT, Vodafone and US-based Verizon, reportedly allowed GCHQ to access undersea cable networks. Four other smaller service providers were also allegedly involved in the covert operation.

The details of the spy programme were first revealed by the Guardian, but the names of the telecom firms have been exposed by Germany's Süddeutsche daily. The newspaper said it has obtained details of a programme codenamed Tempora, in which GCHQ is able to tap and store enormous amounts of user information.

The documents linked to the surveillance programme assign nicknames to all the companies. BT is named "Remedy" while Verizon Business is called "Dacron" and Vodafone Cable "Gerontic".

The firms have so far not responded to the latest disclosures. The latest exposé is bound to have serious ramifications for both GCHQ and Downing Street.

An unidentified source familiar with the matter told the Guardian earlier that the firms do not have any option but to assist GCHQ. In addition, they have been prohibited from divulging details about the operation.

The four smaller companies allegedly involved in the programme are Global Crossing, Level 3, Viatel and Interoute.

All these seven companies put together own tens of thousands of kilometres of fibre-optic network.

Snowden had previously called GCHQ "worse" than its American equivalent NSA in terms of spying capability.

Reports also suggest the firms were rewarded for developing special spying software in order to feed GCHQ with information.