Mobile phone giant Vodafone has revealed that governments around the world use secret wires that allow them to listen to all conversations conducted on its networks.

Some 29 countries in Europe and beyond use the system to monitor phone conversations and track users through their mobile phones.

The company outlined the details in a report on the widespread use of secret surveillance by government agencies.

Vodafone revealed that direct-access wires or pipes were connected directly to its network, the Guardian said. These can allow conversations to be listened to or recorded, or metadata - including the location of a device, the times and dates of communications and with whom communication was made - to be captured.

In six of the countries in which Vodafone operates the wires are a legal requirement, with laws obliging telecommunications companies to install direct-access pipes or allowing governments to do so.

Vodafone is publishing its report to reveal the extent that phone tapping is used by governments to snoop on their citizens, The Guardian said.

The company has called for direct-access pipes to be disconnected and for agencies to have to gain warrants to carry out any surveillance, to discourage them from gaining direct access to a communications network with a legal mandate.

Stephen Deadman, Vodafone's group privacy officer, told The Guardian: "We are making a call to end direct access as a means of government agencies obtaining people's communication data.

"Without an official warrant, there is no external visibility. If we receive a demand we can push back against the agency. The fact that a government has to issue a piece of paper is an important constraint on how powers are used."

Mr Deadman said the use of direct-access pipes in the UK would be illegal because agencies have to obtain a warrant to get information.

The UK issued 2,760 warrants for warrants for the content of calls and messages - rather than metadata - in 2012, the newspaper said, compared with 141,000 made by Italy.

Mr Deadman said: "We need to debate how we are balancing the needs of law enforcement with the fundamental rights and freedoms of the citizens."

Civil rights groups were horrified at the revelations.

Shami Chakrabarti, director of human rights campaign group Liberty, said: "For governments to access phone calls at the flick of a switch is unprecedented and terrifying .

"(Edward) Snowden revealed the internet was already treated as fair game. Bluster that all is well is wearing pretty thin - our analogue laws need a digital overhaul."

Gus Hosein, from Privacy International, said Vodafone had taken a "brave step", calling the covert surveillance wires "the nightmare scenarios that we were imagining".