Home Office ministers have moved to restrict the police's power to snoop on internet surfing and email, to ease the public's fears of 'Big Brother' law enforcement.

Amendments tabled to the Regulation of Investigatory Powers Bill last night would impose stricter conditions on how the police obtain warrants to monitor internet use, Home Office officials said.

Last week, the government said that law enforcement agencies would only be able to decrypt emails where substantial criminal activity was suspected. Otherwise they would only be able to read plain text email intercepted by internet service providers.

Home Office officials said amendments tabled in the House of Lords overnight dealt with the definition of which internet messages the police could read and how far up the chain of command they would need to go to obtain internally authorised and legal warrants for communications data, content or the keys to decode encrypted email.

Changes mean law enforcement agencies could obtain data lists, such as the domains of visited websites, on an internally authorised warrant signed by a police superintendent. However, if the agency wants to know which web pages a suspect has read, and where he or she has clicked onto afterwards - referred to as communications content - the warrant must be signed by the Home Secretary. Encryption keys to decode emails can be obtained on warrants signed by a chief constable.

Opposition peers, however, are said to be uncomfortable over the extent of content the police are still allowed to monitor under an internally authorised warrant. They are likely to push for a requirement that a judge must sign any warrant allowing police to read communications data, such as web site domains, and that a government minister must authorise the handing over of encryption keys.

Caspar Bowden, director of internet policy think tank, the Foundation for Information Policy Research - which has co-ordinated opposition to the Bill in recent weeks - said the definition of what can and cannot be seized under communications data is still in need of further alterations.

"The new definition is still pretty poorly worded, and probably needs further changes to be absolutely sure it excludes the full search-strings submitted to search engines," he said.

The government has reportedly earmarked £20m over four years to subsidise the technological costs of implementing the legislation.

Downing Street officials denied reports that Prime Minister Tony Blair had forced through the latest amendments. "It's been implied that Tony Blair picked up the phone and said to Jack Straw: 'look Jack, you need to sort this out, and to do that you need to do this, this and this'. While that might happen sometimes, it certainly didn't in this case," a spokeswoman for Number 10 told vnunet.com.