The ABC Trial was a trial of charges under sections 1 and 2 of the Official Secrets Act 1911 trial in United Kingdom. It took place in 1978 and is named after the three defendants: Crispin Aubrey, John Berry and Duncan Campbell. Aubrey was a journalist for Time Out, John Berry was a former corporal in signals intelligence (SIGINT), and Duncan Campbell was an investigative journalist.
One of the prosecution witnesses was an anonymous SIGINT officer, referred to as Colonel B. He was later identified as Colonel Hugh Anthony Johnstone, due to his publications in open army journals.
The trial found that the information came almost entirely from open publications, some in the United States. The defendants were found guilty, but no custodial sentences were imposed.