The British spy agency GCHQ may have to delete surveillance data it received from the NSA. Government Communications Headquarters

Millions of people around the world could be able to find out whether their personal data was collected by the U.S. National Security Agency (NSA) and then viewed by Britain's intelligence agency GCHQ, and possibly even have it deleted. A new campaign from human rights advocacy group Privacy International asks people to sign up to find out.

The group will then send the list of people who want to have their details deleted to the U.K. Investigatory Powers Tribunal (IPT), which ruled earlier this month that the data collections were “unlawful.”

The program is called “Did GCHQ illegally spy on you?” and Privacy International is offering sign-ups on its website. The group will then give the list of names to the court, and ask the British intelligence agency to admit whether it had surveillance data on those people.

The IPT ruled on Feb. 6 that data collection from the NSA’s Prism and Upstream programs was unlawful because the rules that determined the collection were kept secret.

“We have known for some time that the NSA and GCHQ have been engaged in mass surveillance, but never before could anyone explicitly find out if their phone calls, emails, or location histories were unlawfully shared between the US and UK,” Eric King, deputy director of Privacy International, said in a statement. "The public have a right to know if they were illegally spied on, and GCHQ must come clean on whose records they hold that they should never have had in the first place.”

The group said it could take several months, and probably even years before any action is taken by intelligence agencies. However, the group said the chance to hold the British spy agency accountable for its misdeeds is a “historic” one.