020

Dialling code for London in the United Kingdom
the United Kingdom telephone area code for London

As is the case for other codes in the UK, the 020 area code may also be used for services without any physical presence in the area, such as private networks or virtual numbering. One such user is the Foreign and Commonwealth Office Telecommunications Network, provided by Global Crossing, 020 is also one of the dialling codes used for telephone services on the remote South Atlantic island of Tristan da Cunha.

Before the introduction of national dialling codes, the area now served by 020 had a large multiple telephone exchange system, called the London telephone area. The first exchange in this area, Central, was opened in the City of London on 1 March 1902. The Director telephone system was developed so that subscribers in London could call one another in a linked numbering scheme regardless of whether they were on a manual local exchange or an automatic one. In 1927, Holborn, the first Director automatic exchange in London, was cut over to the new system at midnight on Saturday 12 November. Because it was mainly a business exchange, most subscribers did not use the new system until Monday 14 November. The successful changeover was delayed by subscribers' unfamiliarity with dialling. Bishopgate and Sloane exchanges followed in six weeks, after which came Western and Monument exchanges. Because the London area contained 80 exchanges, complete conversion took many years. By 1934, the London telephone area comprised all 147 exchanges within 12.5 mile (20 km) of Oxford Circus. By 1950, the London Director system had 75 exchanges within a 5 mile (8 km) radius of Oxford Circus and a further 65 in the 5–10 mile (8–16 km) belt. In Greater London (in other words, within 20 mile (32 km) of Oxford Circus), there were 237 exchanges. In January 1956 a new director exchange – "SKYport" – was opened at London Heathrow Airport.

The London telephone area operated as a single group for charging purposes in which all calls were priced at local rate. Additionally, as was usual, calls to and from the adjacent charge groups were charged as local.

The code serves a roughly circular area with a radius of from the centre of London. The Greater London boundary varies from to from the centre and consequently some outer districts are covered by adjacent codes and in some places the 020 code extends beyond the Greater London boundary. The code covers an area larger than the London post town where letters addressed to "LONDON" are delivered. The City of London and at least some part of all 32 London boroughs are within the 020 area code. There are six exchanges outside Greater London that use the 020 code. Communities outside Greater London that are within the code are Buckhurst Hill, Chigwell, Loughton and Sewardstone in Essex; Borehamwood, Bushey, Carpenders Park, Elstree and South Oxhey in Hertfordshire; and Ewell, Molesey, Thames Ditton and Whyteleafe in Surrey. There are eighteen exchanges within Greater London that do not use the 020 code. The six boroughs that have significant areas within other codes are Bexley, partly within the Dartford (01322) code; Bromley, partly within the Orpington (01689) and Westerham (01959) codes; Croydon, partly within the Orpington (01689) and Redhill (01737) codes; Enfield, partly within the Waltham Cross (01992) code; Havering, mostly within the Romford (01708) code; and Hillingdon, partly within the Uxbridge (01895), Slough (01753) and Watford (01923) codes.