The opposition Labour party on Saturday demanded clarification from the British government about the use by its spies of intelligence from a controversial US surveillance programme.
There are concerns that eavesdropping agency GCHQ, in using US data, may have been able to circumvent British legal processes required to seek personal information from Internet companies.
According to the Guardian newspaper, which helped break news of the existence of the PRISM surveillance programme this week, GCHQ has had access to the US system since 2010 and used it to generate 197 intelligence reports in 2012.
"The intelligence agencies need to be able to get information from abroad especially in their vital work against terrorist threats," said Labour's home affairs spokeswoman Yvette Cooper.
"But it must be within the legal framework agreed by parliament so there are proper safeguards in place."
She called for clarification on what role ministers had in overseeing the use of the US data, and "how this relates to long-standing legal requirements for warrants and inspection by the Intercept Commissioner".
Labour's foreign affairs spokesman Douglas Alexander called on Foreign Secretary William Hague to give a statement to the House of Commons on Monday to address the "serious public concern".
The chairman of parliament's intelligence and security committee, Malcolm Rifkind, has already asked GCHQ to provide a report on its PRISM activities by Monday.
Reports in The Guardian and the Washington Post suggest the US National Security Agency and the FBI are directly tapping central servers at nine US Internet companies to monitor emails, documents, social media posts and online photos.
Dubbed PRISM, the secret programme has been in operation since 2007 and involves media giants Microsoft, Yahoo, Google, Facebook, Paltalk, AOL, Skype, YouTube and Apple, the newspapers said.
The Foreign Office has not responded to the reports about GCHQ's involvement, saying it does not comment on intelligence matters, but Hague is due to give a television interview on Sunday.
GCHQ also refused to comment directly, but insisted in a statement that it operated within a "strict legal and policy framework".
"GCHQ takes its obligations under the law very seriously," the agency said.
The revelations come amid a political debate in Britain about legislation allowing the security services access to emails and so-called media messages.
The so-called "snooper's charter" has split Cameron's coalition his Conservative party supports the proposals but they have been blocked by their junior coalition partners, the Liberal Democrats.