The Local Government (Changes in Electoral Arrangements) Regulations 1985
These Regulations apply where an order made under Part IV of the Local Government Act 1972 which makes substantive changes to the electoral arrangements for a principal area comes into full operation on a day other than one on which an ordinary election of councillors is held. Regulation 2 continues the previous electoral arrangements in force for the purposes of elections to fill casual vacancies until the holding of such an ordinary election. Regulation 3 provides for the re-arrangement of the register of electors in respect of any such election to fill a casual vacancy.