Tisdall anonymously sent The Guardian photocopied documents detailing when American cruise missile nuclear weapons would be arriving in Britain. The documents set out the political tactics Michael Heseltine, then defence minister, would use to present the matter in the House of Commons.
The Government brought a legal action against The Guardian, seeking an order requiring the newspaper to reveal its source. Although The Guardian successfully argued that it was protected by section 10 of the Contempt of Court Act 1981 from providing the information, the judgement by Justice Scott was almost immediately overturned. The appeal by the Attorney General was on the grounds that although the documents themselves were harmless, a civil servant capable of leaking them might leak other documents which could pose a threat to national security.
The Guardian complied with a court order to hand over the documents, which were identified as coming from an FCO photocopying machine. The machine led to Tisdall. In March 1984, Tisdall pleaded guilty to a charge under section 2 of the Official Secrets Act 1911. She was sentenced to six months in jail, but was released after four months.
The legality of the Order (compelling The Guardian to surrender the documents, and thus reveal their source) was upheld in a decision of the House of Lords (Secretary of State for Defence v. Guardian Newspapers Ltd.  AC 339) by a majority of three against two.
- Intermediate-Range Nuclear Forces Treaty
- Clive Ponting was another civil servant who leaked defence information but was acquitted by the jury, who accepted his public interest defence.
- Thomas Andrews Drake and Thomas Tamm leaked information about the American National Security Agency.
- Source protection - information about laws protecting journalists from being compelled to reveal the identity of an anonymous source.
- Journalism source protection
- Parliamentary Question by Tam Dalyell MP
- Peter Preston (5 September 2005). "A source of great regret". The Guardian. Retrieved 29 October 2015.
- Nigel West (2009). The A to Z of British Intelligence. Scarecrow Press. p. 543. ISBN 978-0810868656.
- Roy Greenslade (5 September 2005). "Tisdall's downfall". The Guardian. Retrieved 29 October 2015.