By-elections to the European Parliament in the United Kingdom were held during the period when the United Kingdom used single-member, first-past-the-post constituencies to elect Members of the European Parliament from 1979 to 1999: they were required when a member resigned, died or was disqualified.
The European Parliamentary Elections Act 1999 established a new system of multi-member constituencies elected by a closed party list system based on the regions of England, plus Scotland and Wales.
Because of the new system, retiring MEPs would be replaced by a different person from the list, so no further by-elections were held.
The London South West by-election took place on 20 September 1979, a few months after the 1979 election.
The elected MEP, Shelagh Roberts, was disqualified for holding an office of profit under the Crown.