British intelligence services are able to access data collected by some foreign spy agencies without a warrant, the government has confirmed for the first time.

The agreement has been revealed in a document submitted by GCHQ in response to a legal challenge by privacy and human rights organisations in the wake of the classified information revealed by Edward Snowden last year.

The document reads: "[A] warrant is not as a matter of law required in all cases in which unanalysed intercepted communications might be sought from a foreign government."

It "essentially permit[s] bulk collection of material, which can include communications of UK citizens", explains The Guardian's James Ball.

Once obtained, the data can be kept for up to two years, or longer if a senior intelligence official believes it to be necessary for national security.

This goes against the government's comments last year when parliament's intelligence and security committee assured the public that a warrant signed by a minister was needed before GCHQ requested information from US agencies.

Privacy International, one of the groups involved in taking legal action against GCHQ and NSA said the revelation will cause further distrust in the UK's surveillance methods.

"We now know that data from any call, internet search, or website you visited over the past two years could be stored in GCHQ's database and analysed at will, all without a warrant to collect it in the first place," said deputy director Eric King.

"It is outrageous that the government thinks mass surveillance, justified by secret 'arrangements' that allow for vast and unrestrained receipt and analysis of foreign intelligence material is lawful."

American privacy advocates were also quick to condemn the warrantless access of US data. British and US intelligence agencies are "circumventing even the very weak safeguards that have been put in place," Jameel Jaffer, deputy legal director of the American Civil Liberties Union told the National Journal. "It underscores both the inadequacy of existing oversight structures and the pressing need for [surveillance] reform."