Taxpayers' personal data, including records of web sites they have visited and where their mobile phone calls are made, is being viewed by Her Majesty's Revenue and Customs on an increasingly frequent basis.

In 2011, HMRC was authorised to view 14,381 items of "communications data" on taxpayers while investigating tax evasion, compared with 11,513 items in 2010, according to figures released under the Freedom of Information Act and seen by The Independent.

Using the Regulation of Investigatory Powers Act 2000, HMRC can see web sites viewed by taxpayers; where a mobile phone call was made or received; and the date and time of emails, texts and phone calls. It is not clear how many times the surveillance has led to a successful prosecution for tax evasion – or whether those found to be innocent are told that they have been spied on. HMRC did not respond to requests for this information.

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Stephen McPartland, the Conservative MP for Stevenage who is campaigning for large companies to be more open about tax, said:"HMRC should be focusing on the estimated £35bn lost tax, not snooping on hard-working people," he said.

Although there is broad public support for the Government's crackdown on tax evasion, the scale of HMRC's use of surveillance powers is likely to raise concerns that it is infringing people's privacy without being properly accountable.

From October 2011 to the end of September 2012, HMRC was given 172 authorisations for "directed surveillance" – covert surveillance, mainly in public places – down slightly from the previous year.

HMRC refused to disclose how many times it had been given warrants to intercept and read peoples' private emails, or listen to their phone calls. This is the most intrusive type of surveillance, which needs to be authorised by the Home Secretary. It also refused to disclose the number of times it had carried out "intrusive surveillance", which can include covertly filming a person's house, or bugging their property or car.

Adam Craggs, a partner and tax specialist at law firm RPC, said: "It is not immediately apparent why such communications data would be useful to HMRC for the purpose of tax investigations. Why does HMRC require details of the nature of web sites visited, which may be perfectly legal but potentially embarrassing, such as dating sites?"

Rachel Robinson, a policy officer at Liberty, said that while surveillance was vital when investigating serious crimes, "highly personal data is already too widely available and over-broad powers are regularly used and abused."

An HMRC spokesman said: "Tackling serious organised crime is a priority for us and access to communications data and directed surveillance have a vital role to play in meeting that challenge.

"In 2011 communications data enabled us to prevent £850m of tax revenue being diverted into the pockets of fraudsters. Our use of these powers is subject to regular independent inspection, ensuring it is both proportionate and lawful."

Surveillance state

Ripa, as the Act is known, allows police forces, local authorities, government agencies and public bodies to use surveillance including tapping phones, opening mail, bugging or secretly filming suspects, and accessing phone and computer records. It can be employed by organisations ranging from the BBC and Ofsted to the Royal Mail. There is no legal obligation for an individual to be informed that they are under surveillance. The Protection of Freedoms Act now requires councils to gain a magistrate's approval to use the powers.