Edward Snowden leaks: UK 'let NSA store email addresses'
The UK allowed the US National Security Agency to keep the mobile phone numbers and email addresses of ordinary Britons from 2007, reports say.
The reports are based on documents leaked by whistleblower Edward Snowden.
The Foreign Office said it did not comment on such "speculation".
Under the seven-decade-old UK-US agreement on intelligence sharing, Britain and America's intelligence agencies are not supposed to collect information on each other without permission.
Before 2007, if the US - on one of its other operations - had come across a British mobile phone number or email address, it would not have been able to keep it.
But, according to the documents, after the rules changed, the US was allowed to keep those records to analyse them for the specific purpose of "contact chaining".
That means looking at the connections between different phone numbers or email addresses.
BBC security correspondent Gordon Corera said it was not quite the same as reading emails or listening to mobile phone calls, which still requires a warrant.
But he said the documents raised questions about the motive behind the decision and to what extent the public had a right to know about such programmes.
It is also claimed that a 2005 memo raised the possibility of the US carrying out surveillance on British nationals without British permission when it was considered to be in America's national interest.
But it is not clear if that proposal was acted on.
The Foreign Office said in a statement it did not comment on "speculation around intelligence matters".
"If you are a terrorist, a serious criminal, a proliferator, a foreign intelligence target or if your activities pose a genuine threat to the national or economic security of the United Kingdom, there is a possibility that your communications will be monitored," the statement added.
"If you are not, and if you are not in contact with one of those people, then you won't be. That is true, actually, whether you are British, if you are foreign and wherever you are in the world."
Former NSA contractor Edward Snowden fled to Russia in June after leaking details to the media of extensive internet and phone surveillance by American intelligence.
Mr Snowden, who has been granted temporary asylum in Russia, is wanted by the US on espionage charges over his actions.