An Irish Republican Army veteran charged with the IRA killing of four soldiers in London's Hyde Park in 1982 has been freed from custody after a judge ruled that the man had received secret British assurances he would not be arrested.

The judge said British authorities in Northern Ireland in 2007 provided John Downey with a confidential document promising him he would not be arrested if he entered the United Kingdom.

The 62-year-old Downey, who lives in the Republic of Ireland, was arrested last year at London's Gatwick Airport and charged with four counts of murder over the Hyde Park bomb.

Tuesday's judgment found that the 2007 document issued to Downey provided gave false assurance, because London police still wanted to arrest him.