Hornchurch was a borough constituency represented in the House of Commons of the Parliament of the United Kingdom. It elected one Member of Parliament (MP) by the first past the post system of election. At the 2010 general election parts formed the new seats of Hornchurch and Upminster; and Dagenham and Rainham.

1945–1974: The Urban District of Hornchurch.

1974–1983: The London Borough of Havering wards of Elm Park, Hacton, Hylands, Rainham, St Andrew's, and South Hornchurch.

1983–2010: The London Borough of Havering wards of Airfield, Elm Park, Hacton, Hylands, Rainham, St Andrew's, and South Hornchurch.^[1][2]

The seat established in 1945 covered the Hornchurch Urban District, which had been enlarged in the 1930s to include the civil parishes of Cranham, Great Warley, Rainham, Upminster and Wennington in addition to the parish of Hornchurch, which included the neighbourhoods of Ardleigh Green, Elm Park, Emerson Park, South Hornchurch and Harold Wood. The population of the urban district in 1939 was 81,486. The most populous Hornchurch North West ward was very near to the town of Romford. The area became part of the London Borough of Havering in Greater London in 1965.

In 1974 the seat was redrawn, transferring Cranham, Emerson Park, Great Warley, Harold Wood and Upminster to the new Upminster constituency. The constituency shared boundaries with the Hornchurch electoral division for election of councillors to the Greater London Council at elections in 1973, 1977 and 1981. The electoral wards in Havering were redrawn in 1978, with the constituency defined in reference to the new wards from 1983. The new Airfield ward reflected the building of the Airfield Estate housing development on land that had been RAF Hornchurch.

The constituency was abolished for the 2010 election. The areas of the constituency covered by the Elm Park, South Hornchurch, and Rainham and Wennington wards in the London Borough of Havering were merged with Dagenham to form a cross-borough Dagenham and Rainham constituency. Hacton and St Andrews wards in Hornchurch merged with Upminster to form Hornchurch and Upminster. Hylands ward in Hornchurch merged with Romford. Prior to the change in boundaries the new seats were predicted to be marginal Labour and safe Conservative respectively if they followed the voting patterns of the previous Dagenham, Upminster and Romford constituencies.^[3]

The south of the constituency was the industrialised Hornchurch Marshes, which included the eastern part of the Ford Dagenham plant. The adjacent Rainham Marshes had been seen as a site for building large entertainment centres and was viewed as a potential site for the European Disneyland project, although it was considered much less suitable than the current position near Paris. There had been plans to build a casino but permission was not granted.

Hornchurch is a predominantly suburban and residential area. The constituency was a marginal seat due to Rainham and Elm Park's working class voters and because the wealthiest Emerson Park area of Hornchurch did not formed part of the constituency after 1974.

The Conservative Robin Squire was elected to Parliament as the member for Hornchurch on 3 May 1979, in one of the most surprising results of that election. Labour-held Hornchurch had not been a marginal seat and Squire had not expected to win it. However, he gained the seat from Alan Lee Williams with a majority of just 769 on a "freak" swing of 8.5% to his party. During the Thatcher years (1979 to 1990) Squire was considered to be a prominent "wet", opposed to the Conservative government's economic and employment policies. After Mrs Thatcher left office in 1990, Squire's political position strengthened and he held junior ministerial posts until the fall of the Major government in 1997. Squire was defending a majority of 9,165 – his personal popularity plus his prominence as a Minister led him to believe that he would hold the seat, but he lost to Labour's John Cryer with a 16% swing and a Labour majority of 5,680. Squire stood against Cryer again in the 2001 general election but was again defeated by a significant majority.

• List of parliamentary constituencies in London