The Home Office has awarded new powers to the police and MI5 allowing them to hack into personal computers without a warrant. The move follows proposals by the European Union which extend the use of intrusive surveillance.

The remote searching technique uses keyloggers, which can be installed in a variety of ways to allow officers to monitor a suspect's computer usage, including emails, web surfing and instant messaging conversations.

Each case must still be authorised by a chief constable, but the new measures have angered opposition MPs, as well as civil liberty and privacy organisations, which are threatening a legal challenge to the extension.

"The exercise of such intrusive powers raises serious privacy issues," said shadow home secretary Dominic Grieve. "The government must explain how they would work in practice and what safeguards will be in place."

Shami Chakrabati, director of human rights group Liberty, added: "These are very intrusive powers. The public will want this to be controlled by new legislation and judicial authorisation.

"Without those safeguards, it is a devastating blow to any notion of personal privacy. This is no different from breaking down someone's door, rifling through their paperwork and seizing their computer hard drive."

The Association of Chief Police Officers (ACPO) defended the move, pointing out that it would still be governed by the Regulation of Investigatory Powers Act.

"To be a valid authorisation, the officer giving it must believe that it is necessary to prevent or detect serious crime ... and is proportionate to what it seeks to achieve," said an ACPO spokesman.

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

According to the police, 194 police hacking operations were carried out in England, Wales and Northern Ireland over the past two years: 133 in private homes, 37 in offices and 24 in hotels.