Basildon was a parliamentary 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.

The seat was created for the February 1974 general election from the majority of the abolished constituency of Billericay. Its electorate was reduced for the 1983 general election when Billericay was re-established. It underwent a major redistributions for the 1997 election and again for the 2010 election, when it was renamed as South Basildon and East Thurrock.

Basildon was one of the best known bellwether constituencies in Britain, having voted for the winning party in each election for the duration of its existence. The failure of the Labour Party to win it in the 1992 election — Basildon would be the first to declare of all the marginal seats — foreshadowed Labour's fourth straight defeat at the hands of the Conservative Party.

It was a (famously) Conservative seat between 1979 and 1997 as the quintessential "Essex man" constituency based on the new town of Basildon. Much of its population has its origins in the East End of London, who as a group traditionally voted for the Labour Party.

However, throughout the 1980s and until the Labour landslide victory of 1997, many voters in the area transferred their loyalties to the Conservative Party. The MP throughout that period was David Amess, who himself originated from east London (Plaistow), and later held the safer seat of Southend West. New Labour sought to take seats exactly like this one, which were crucial in ensuring they won the 1997 general election by a landslide. Labour took the seat in 1997 and 2001 with a large majority, and in 2005 with a reduced majority.

Both the successor seats that replaced Basildon in 2010 were considered to be much less favourable to Labour than the old Basildon seat, ending Basildon's bellwether status.

• The Urban District of Basildon.

Formed from the majority of the abolished constituency of Billericay, comprising the former Urban District thereof, which had been reconstituted as the Urban District of Basildon.

• The District of Basildon wards of Fryerns Central, Fryerns East, Langdon Hills, Lee Chapel North, Nethermayne, Pitsea East, Pitsea West, and Vange.

Northern parts, including Billericay and Wickford transferred to the re-established constituency of Billericay.

• The District of Basildon wards of Fryerns Central, Fryerns East, Langdon Hills, Lee Chapel North, Nethermayne, and Vange; and
• The Borough of Thurrock wards of Corringham and Fobbing, Orsett, Stanford-le-Hope, and The Homesteads.

Major realignment of boundaries with Billericay; Pitsea was transferred to Billericay in exchange for the northern part of the Borough of Thurrock.

Following their review of parliamentary representation in Essex, the Boundary Commission for England made radical changes to all parliamentary constituencies in the county for the 2010 general election, and the Basildon constituency was divided. The majority of the seat formed the basis of the new South Basildon and East Thurrock constituency, while the wards around central Basildon were merged with the majority of the former Billericay constituency to form a new seat called Basildon and Billericay.

Election		Member	Party
	February 1974	Eric Moonman	Labour
	1979	Harvey Proctor	Conservative
	1983	David Amess	Conservative
	1997	Angela Smith	Labour Co-op
	2010	constituency abolished: see South Basildon and East Thurrock and Basildon and Billericay

General election February 1974: Basildon

Party		Candidate	Votes	%	±%
	Labour	Eric Moonman	33,499	45.2
	Conservative	Ronald Denney	22,832	30.8
	Liberal	Edward Fortune	17,794	24.0
Majority			10,667	14.4
Turnout			74,125	82.0
	Labour win (new seat)

General election October 1974: Basildon

Party		Candidate	Votes	%	±%
	Labour	Eric Moonman	32,298	47.9	+2.7
	Conservative	David Atkinson	21,747	32.2	+1.4
	Liberal	Edward Fortune	12,816	19.0	―5.0
	Ind. Labour Party	Robert Chaplin	599	0.9	New
Majority			10,551	15.6	+1.2
Turnout			67,460	73.8	―8.2
	Labour hold		Swing	+0.6

General election 1979: Basildon

Party		Candidate	Votes	%	±%
	Conservative	Harvey Proctor	37,919	46.9	+14.7
	Labour	Eric Moonman	32,739	40.5	―7.4
	Liberal	Raymond Auvray	9,280	11.5	―7.5
Majority			5,180	6.4	N/A
Turnout			79,938	78.0	+5.2
	Conservative gain from Labour		Swing	+11.05

Note: This constituency underwent boundary changes after the 1979 election, so was notionally a Labour seat.

General election 1983: Basildon

Party		Candidate	Votes	%	±%
	Conservative	David Amess	17,516	38.7	+3.6
	Labour Co-op	Julian Fulbrook	16,137	35.6	―17.0
	SDP	Sue Slipman	11,634	25.7	+14.4
Majority			1,379	3.0	N/A
Turnout			45,287	69.0	―9.0
	Conservative gain from Labour		Swing	+10.3

General election 1987: Basildon

Party		Candidate	Votes	%	±%
	Conservative	David Amess	21,858	43.5	+4.8
	Labour Co-op	Julian Fulbrook	19,209	38.3	+2.7
	Liberal	Raymond Auvray	9,139	18.2	―7.5
Majority			2,649	5.3	+2.3
Turnout			50,206	73.8	+4.8
	Conservative hold		Swing	+1.1

General election 1992: Basildon

Party		Candidate	Votes	%	±%
	Conservative	David Amess	24,159	44.9	+1.4
	Labour Co-op	John Potter	22,679	42.2	+3.9
	Liberal Democrats	Geoffrey Williams	6,967	12.9	―5.3
Majority			1,480	2.7	―2.6
Turnout			53,805	79.8	+6.0
	Conservative hold		Swing	―1.3

General election 1997: Basildon

Party		Candidate	Votes	%	±%
	Labour Co-op	Angela Smith	29,646	55.8	+15.1
	Conservative	John Baron	16,366	30.8	―14.2
	Liberal Democrats	Lindsay Granshaw	4,608	8.7	―6.5
	Referendum	Craig Robinson	2,462	4.6	New
Majority			13,280	25.0	N/A
Turnout			53,082	72.0	―7.6
	Labour Co-op gain from Conservative		Swing	+14.7

General election 2001: Basildon

Party		Candidate	Votes	%	±%
	Labour Co-op	Angela Smith	21,551	52.7	―3.1
	Conservative	Dominic Schofield	13,813	33.8	+3.0
	Liberal Democrats	Jane Smithard	3,691	9.0	+0.3
	UKIP	Frank Mallon	1,397	3.4	New
	Socialist Alliance	Dick Duane	423	1.0	New
Majority			7,738	18.9	―6.1
Turnout			40,875	55.1	―16.9
	Labour Co-op hold		Swing	―3.0

General election 2005: Basildon

Party		Candidate	Votes	%	±%
	Labour Co-op	Angela Smith	18,720	43.4	―9.3
	Conservative	Aaron Powell	15,578	36.1	+2.3
	Liberal Democrats	Martin Thompson	4,473	10.4	+1.4
	BNP	Emma Colgate	2,055	4.8	New
	UKIP	Alix Blythe	1,143	2.6	―0.8
	Green	Vikki Copping	662	1.5	New
	English Democrat	Kim Gandy	510	1.2	New
Majority			3,142	7.3	―11.6
Turnout			43,141	58.4	+3.3
	Labour Co-op hold		Swing	―5.8

• List of parliamentary constituencies in Essex

• Basildon UK Parliament constituency (boundaries April 1997 – April 2010) at MapIt UK

51°31′N 0°24′E / 51.52°N 0.40°E / 51.52; 0.40