Francis Walsingham, who ran Elizabeth I’s secret service, built up a vast spy network to protect the Queen against the very real risk of assassination.
His greatest coup was to intercept and decode letters exchanged in 1586 between Mary, Queen of Scots, and a group of Catholics led by Anthony Babington, which led to their executions.
In 1794, the government became suspicious about a discussion group called the London Corresponding Society, whose members were showing too much interest in the progress of the French revolution. A spymaster named William Wickham filled the LCS with informers, but failed to find any evidence of treasonable contact with French revolutionaries. The only weapons they found were in the home of a disgruntled government spy. William Pitt nonetheless used this flimsy evidence to suspend habeas corpus.
After the Irish patriot, Sir Roger Casement, was hanged in 1916 for his role in the Easter rising, extracts from his private diaries were circulated to discredit him by revealing that he was promiscuously gay. It was suspected for a long time that they were forgeries. In fact, British intelligence had genuinely got hold of a man’s private diaries and used them to sully his reputation.
MI5 is believed to have had a high level informant in the miners’ union during the 1984-5 miners’ strike, though his identity has never been uncovered. But it is known that Harry Newton, who was active in the unions and CND from the 1950s until his death in 1983 was a long time MI5 informant. The ex-spy who exposed him, Cathy Massiter, also revealed that MI5 was listening into the telephone conversations of the young future Cabinet ministers, Patricia Hewitt and Harriet Harman.
Sometimes, however, the spooks look but do not see. In 2004, they were watching a young Muslim named Omar Khyam, whom they had identified as a leader of a terrorist cell, and saw him talking to two “unidentified males”. It later transpired that the pair were Mohammed Sidique Khan and Shehzad Tanweer, the July 7 2005 bombers.