Police have been given the power to hack into personal computers without a court warrant. The Home Office is facing anger and the threat of a legal challenge after granting permission. Ministers are also drawing up plans to allow police across the EU to collect information from computers in Britain.

The moves will fuel claims that the Government is presiding over a steady extension of the "surveillance society" threatening personal privacy.

Hacking – known as "remote searching" – has been quietly adopted by police across Britain following the development of technology to access computers' contents at a distance. Police say it is vital for tracking cyber-criminals and paedophiles and is used sparingly but civil liberties groups fear it is about to be vastly expanded.

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Remote searching can be achieved by sending an email containing a virus to a suspect's computer which then transmits information about email contents and web-browsing habits to a distant surveillance team.

Alternatively, "key-logging" devices can be inserted into a computer that relay details of each key hit by its owner. Detectives can also monitor the contents of a suspect's computer hard-drive via a wireless network.

Computer hacking has to be approved by a chief constable, who must be satisfied the action is proportionate to the crime being investigated.

Last month European ministers agreed in principle to allow police to carry out remote searches of suspects' computers across the EU.

Details of the proposal are still being developed by the Home Office and other EU ministries, but critics last night warned it would usher in a vast expansion of police hacking operations.

Shami Chakrabarti, director of the human rights campaign group Liberty, said such a vast expansion of police powers should be regulated by a new Act of Parliament and that police should be forced to apply to a court for a warrant to hack into computers.

She said: "This is no different from breaking down someone's door, rifling through their paperwork and seizing their computer hard drive."

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Ms Chakrabarti said the organisation believed it had strong grounds to challenge the practice both under British and European law.

Dominic Grieve, the shadow Home Secretary, said: "The exercise of such intrusive powers raises serious privacy issues. The Government must explain how they would work in practice and what safeguards will be in place."

A spokesman for the Association of Chief Police Officers, said police carried out 194 hacking operations in 2007-08 in England, Wales and Northern Ireland, including 133 in private homes, 37 in offices and 24 in hotel rooms.

The spokesman said such surveillance was regulated under the Regulation of Investigatory Powers Act.

"The police service in the United Kingdom will aggressively pursue serious and organised criminality, including where that takes the modern forms of hi-tech crime," he added.

The Government faces criticism over the erosion of civil liberties on a series of fronts. It is working on plans for a giant "big brother" database holding information about every phone call, email and internet visit made by everyone in the United Kingdom.

The first Britons will receive biometric identity cards at the end of the year, paving the way to the world's largest identity register. Genetic details of more than four million people are on the DNA national database, the highest proportion of any Western country. The European Court of Human Rights has ruled that Britain's policy of retaining samples from people never convicted of a crime – including children – breaches human rights.