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The UK allowed the US National Security Agency to keep the mobile phone numbers and email addresses of ordinary Britons from 2007, the BBC reported, citing reports.

Channel 4 News and the Guardian said existing rules were modified in 2007 to allow the US to keep information swept up incidentally about Britons not suspected of criminal activity.

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.