Police Oracle have just posted an excellent article on case law surrounding Covert Human Intelligence Sources (CHIS) or Confidential Informants in the United Kingdom. Most importantly, the case law covers the extent to which CHIS can/cannot encourage others to participate in criminal offences. According to R v Mealey (1974) it is unacceptable for a CHIS to entice another person to commit an offence which he/she would not otherwise have committed. The article goes on to cite Teixeira de Castro v Portugal (1999) and R v Moon (2004) in which the accused were encouraged to take part in illegal activity.
In America, entrapment by Confidential Informants is often used by defence lawyers. Examples include a case of dog fighting charges in Indianapolis and a case of robbery in Texas in September 2010, the Florida case against John Nicholas Coors in a murder-for-hire plot in April 2010 and the August 2010 New York terror trial.