Chiltern Hundreds

Under a Resolution of the House of 2 March 1623, Members of Parliament cannot directly resign their seat. Death, disqualification, elevation to the Peerage, dissolution or expulsion are the only causes by which a Member's seat can be vacated. Therefore a Member wishing to resign has to go through the process of applying for a paid office of the Crown, which automatically disqualifies the Member from holding a seat in the House of Commons. There are two such offices: Crown Steward and Bailiff of the Chiltern Hundreds and the Manor of Northstead.

See this House of Commons factsheet for further information: http://www.parliament.uk/commons/lib/p11.pdf