Want to know if GCHQ spied on you? Now you can find out. Privacy International (PI) has just launched a website that lets anyone find out if their communications were intercepted by the NSA and then shared with GCHQ.

The website is the result of a February 6 ruling by the Investigatory Powers Tribunal (IPT). Similar to the Foreign Intelligence Surveillance Court in the US, the IPT is a special court in the UK established by the Regulation of Investigatory Powers Act (RIPA) that deals with issues of surveillance and human rights.

The February 6th ruling held that intelligence sharing between GCHQ and NSA done prior to December 2014 was unlawful. The decision, which applied to information collected by the NSA through Prism and Upstream, was based on the secrecy of the rules governing sharing of that information. This followed a December ruling in which the court held that information sharing between the NSA and GCHQ could continue because the oversight of the data-collection program had been made public, bringing it into compliance with European law. Privacy International disagreed with the decision made by the tribunal on this point and is appealing to the European Court of Human Rights.

As Privacy International points out, “The [February] decision was the first time in the Tribunal’s history that it had ruled against the actions of the intelligence and security services.”

But what does the ruling mean? That’s where Privacy International comes in.

Because of the ruling, there is an opportunity for people to try to find out if their communications were among those shared by NSA with GCHQ. Intelligence agencies use information like ip addresses and email addresses as “selectors” when sifting through the massive quantities of data they collect. GCHQ will comply with the IPT’s ruling by searching “selectors” it received from the NSA prior to December 2014.

But this won't happen automatically. People need to actually file requests with the IPT. To help people do so, PI is collecting people’s names, numbers, and emails in order to assist them in asserting their rights and finding out whether those selectors were subject to unlawful sharing. If they were, PI will help individuals seek a declaration that that person’s privacy rights have been violated under Article 8 and Article 10 of the UK Human Rights Act, the law that codified the European Convention on Human Rights into UK law. Once the IPT issues a declaration for an individual, that individual can also request that their records be deleted. There’s no need to be a UK citizen—anyone can participate.

One thing to note: In order to determine who is affected, PI will have to pass selectors on to IPT and GCHQ. PI notes that “GCHQ are only allowed to keep your details for the purposes of establishing whether or not they spied on you illegally and for the duration of the investigation by the IPT.” But those whose selectors (for instance, a phone number and a legal name) are not publicly associated with each other may want to be careful, because the GCHQ would now know that they are associated.

But the risk is relatively low, and the payoff is that the more people who sign on and learn that they’ve been affected by GCHQ and NSA spying, the clearer it becomes that reform to surveillance is urgently needed. Eric King, Deputy Director of Privacy International, put it best:

There are few chances that people have to directly challenge the seemingly unrestrained surveillance state, but individuals now have a historic opportunity [to] finally hold GCHQ accountable for their unlawful actions.

To participate in Privacy International’s campaign, sign up here. To learn more, read PI’s FAQ here.