If legal processes to obtain data on UK citizens are being circumvented, it is a "very serious issue" and questions have to be answered at the highest levels.
Following the revelation on Friday that the UK government has been using a US-run program which gives operatives unhindered access to data on the servers of companies like Google, Facebook and Apple, privacy advocates have been looking for answers from the government.
Nick Pickles, director of privacy and civil liberties campaign group Big Brother Watch, said: "Surveillance without suspicion is an affront to a free society and it is essential that we get to the bottom of the NSA and US Government has being doing and how British citizens may have seen their privacy compromised on a vast scale."
The comments follow the revelation on Friday that Government Communications Headquarters (GCHQ) has been using a US-run programme called Prism which gives agents access to data such as email, video, photos, instant messagi and voice-over-IP calls (such as Skype).
The programme has been in use by the National Security Agency (NSA) in the US since 2007, according to documents leaked to the Guardian and the Washington Post on Thursday.
Pickles believes that in order to protect British citizens' privacy, answers will have to be forthcoming about just what powers the government has: "If British citizens have had their emails and social media messages seized by the US Government without any justification or legal authority, serious questions must be asked at the highest levels."
Commenting on leaks, the Government Communications Headquarters (GCHQ) said: "Our work is carried out in accordance with a strict legal and policy framework which ensures that our activities are authorised, necessary and proportionate, and that there is rigorous oversight, including from the secretary of state, the interception and intelligence services commissioners and the intelligence and security committee."
Typically the UK government can request information about people using services like Google and Facebook through diplomatic channels by making individual requests on a case-by-case basis to the US Department of Justice, who in turn would make the request on its behalf to the company involved. For example in 2012, the UK government made 3,000 requests to Google alone.
Pickles believes that storing data in the cloud is making it easier for governments around the world to spy on their - and other countries' - citizens: "The revelations call into question the integrity of cloud services that are used by millions of non-US citizens every day, while setting a dangerous precedent that less-democratic regimes around the world may rush to copy. How many Members of Parliament, business leaders and key security figures use US-based services that may have been compromised?"