Supergrass is a British slang term for an informer, which originated in London. Informers had been referred to as "grasses" since the late-1930s, and the "super" prefix was coined by journalists in the early 1970s to describe those informers from the city's underworld who testified against former associates in a series of high-profile mass trials at the time. One of the first police informers to receive the 'supergrass' nickname was Bertie Smalls.
Usage in Northern Ireland
In Northern Ireland, the term "supergrass" especially refers to arrested paramilitaries who divulged the identities of their compatriots to the Royal Ulster Constabulary, possibly in exchange for immunity from prosecution. Sir John Hermon did not deny reports that inducements were paid but denied figures as high as £50,000 were involved. The use of the term in Northern Ireland began with the arrest of Christopher Black in 1981. After securing assurances that he would have protection from prosecution, Black gave statements which led to 38 arrests. On 5 August 1983, 22 members of the Provisional IRA were sentenced to a total of more than 4,000 cumulative years in prison, based on Black's testimonies alone (eighteen of these convictions were overturned on appeal on 17 July 1986).
By the end of 1982, 25 more 'supergrasses' had surfaced contributing to the arrests of over six hundred people from paramilitary organizations, such as the Provisional IRA, the Irish National Liberation Army (INLA) and the Ulster Volunteer Force.
On 11 April 1983, members of the loyalist Ulster Volunteer Force were jailed on the evidence of supergrass Joseph Bennett. These convictions were all overturned on 24 December 1984. In October 1983, seven people were convicted on the evidence provided by supergrass Kevin McGrady although the trial judge Lord Chief Justice Robert Lowry had described McGrady's evidence as "bizarre, incredible and contradictory". The last supergrass trial finished on 18 December 1985, when 25 members of the INLA were jailed on the evidence of Harry Kirkpatrick. 24 of these convictions were later overturned on 23 December 1986.
Many convictions based on supergrass testimony were later overturned, and the supergrass system was discontinued in 1985 until recently in 2011. The first supergrass trial in 26 years began on the 8th of September 2011 for the murder of UDA member Tommy English.
The term has been used more recently to describe an informant with al-Qaeda links testifying at the trials for seven British men conspiring to cause explosions between 1 January 2003 and 31 March 2004, and again on 2 July 2007 in an article in the Daily Mail describing a search for informants in the 2007 Glasgow International Airport attack.
The term has also been used by The Royal Gazette, a daily newspaper in Bermuda, a British dependent territory. An article in the paper uses the term to describe a Transport Control Department worker convicted of selling driver's licenses to Portuguese applicants lacking the necessary English skills to pass the multiple choice exam. The worker was granted a conditional discharge in exchange for information on other Transport Control Department employees abusing the public trust.
One of the most prolific supergrasses in recent British history was Michael Michael whose evidence in 2001 led to 32 criminals being convicted, including his own mother, and the disruption of a £132million drugs ring. 
- Jackson, John D. "THE JURY SYSTEM IN CONTEMPORARY IRELAND: IN THE SHADOW OF A TROUBLED PAST."
- Informers crippling IRA...; The Times; 25 Mar 1982; pg1 col E
- 1983: IRA members jailed for 4,000 years
- Fortnight: An Independent Review For Northern Ireland, p.5, 2–15 June 1986 edition
- "'Terror ringleader' is brilliant NHS doctor", 27 Aug 2007[dead link]
- TCD worker gave exam passes to Portuguese drivers for bribes. 06 Oct 2009
- Britain's biggest supergrass
- Supergrasses: The Use of Accomplice Evidence in Northern Ireland; Tony Gifford ISBN 0-900137-21-5
- The Crowned Harp: Policing Northern Ireland — by Graham Ellison and Jim Smyth (2000)
- Supergrass tells of terror fight — BBC News article
- Kevin Morrison Liverpool
- Kevin Morrison Police informant jailed for life Liverpool