The court ruled the rules had been breached
The European Court of Human Rights has ruled Britain breached international conventions by monitoring emails and phone calls between Ireland and the UK.
The data was intercepted over a seven year period from 1990 to 1997.
The court found the surveillance was in breach of a convention guaranteeing respect for private correspondence.
The case was taken by three organisations, the Irish Council for Civil Liberties, Liberty and British-Irish Rights Watch.
For security reasons, the UK neither confirmed nor denied the statements made about its surveillance activities, but it agreed that the court could presume some of the civil liberties' groups communications were intercepted.
"The court recalled that it had previously found that the mere existence of legislation which allowed communications to be monitored secretly had entailed a surveillance threat for all those to whom the legislation might be applied," the court said in a statement.
"In the applicants' case, the court therefore found that there had been an interference with their rights as guaranteed by Article 8," the court said referring to the article in the European Charter of Human Rights on the right to privacy.
The 1985 Interception of Communications Act gave British authorities "extremely broad discretion" to intercept communications between Britain and another country, it added.
"The court considered that the domestic law at the relevant time had not indicated with sufficient clarity, so as to provide adequate protection against abuse of power," it added.