Private members' bills, not to be confused with private bills, are public bills introduced to the UK Parliament by back‐bench MPs or peers. They may be on any issue as long as the Crown's sole prerogative to propose public expenditure is not breached. In the Commons they may be introduced by the first twenty MPs drawn in a ballot for each parliamentary session, for whom time is allocated on Fridays when they are given precedence over government bills. They may also be introduced under the ‘ten minute rule’ at prescribed times during normal parliamentary business. Few of the latter achieve enactment, although success is more generally measured in the extent to which parliamentary and public attention has been drawn to the subject of the bill. Bills introduced by MPs successful in the ballot have a better chance of enactment, although many still fail for lack of parliamentary time. It is crucial that MPs presenting ballot bills do not excite the opposition of the government, so as to ensure assistance from government departments in preparing them, and to guard against fatal parliamentary opposition. They must also lobby support in both Houses.
Private members' bills have frequently aroused controversy, particularly in the 1960s when they resulted in legislation on contraception, homosexuality, abortion, and divorce. Opponents suggest that MPs do not have an electoral mandate to introduce bills and debates are far too rushed to deal with weighty moral issues. Advocates stress that it is in private members' bills that Parliament, sovereign and acting as a legislature independent of the executive, lives on. More pragmatically, private members' bills may be seen as vehicles to present legislation on matters for which the Government cannot find a place in its own parliamentary timetable, or upon which there is substantial social consensus outside Parliament but problematic dissenting voices in the governing party. Recent legislation resulting from private members' bills includes the Female Genital Mutilation Act, 2003, the Gangmasters Licensing Act, 2004, and the Sustainable Communities Act, 2006.
http://www.parliament.uk/about/how/laws/private_members.cfm Parliament site for private members' bills.