The British Rail Class 700 is an electric multiple unit passenger train from the Desiro City family built by Siemens Mobility.

Both the procurement process and final close of contract were significantly delayed, resulting in the expected first delivery date moving from 2012 to 2016.

[9] Ride quality and noise levels were expected to equal or be better than those of current vehicles and climate control (air-conditioning) was to be fitted.

[11] The vehicles were to be fitted for driver-only operation, and to include GSM-R communications radio, as well as AWS, TPWS, and ERTMS level 2 safety systems.

[11] In July 2008, the Department for Transport shortlisted consortia including Alstom, Bombardier, Hitachi, and Siemens as train builders.

[17] Bombardier Transportation offered the Aventra, a design incorporating a development of the FLEXX Eco inside frame bogie with bogie-mounted traction motors.

[18] Both Bombardier's and Siemens' rolling-stock designs were conventional EMUs incorporating inside frame bogies and modern passenger and rolling stock information systems.

[9] The award of the contract was delayed by the 2010 general election[19] and the subsequent spending review, following which the procurement was announced to be proceeding in late 2010.

The vehicles would be manufactured at Siemens' plant in Krefeld, Germany, and maintenance depots were to be built at Hornsey (London) and Three Bridges (Sussex).

[23][24][25][26] In mid-2013 the National Audit Office (NAO) reported that the contract delay could negatively impact the delivery of the entire Thameslink Programme.

[27] The £1.6 billion contract to finance, supply, and maintain a 1,140-carriage fleet of passenger rolling stock was eventually finalised between the DfT, the supplier Siemens, and the Cross London Trains consortium on 14 June 2013.

[36] A mock-up of the train was unveiled at the ExCel centre in January 2014, and then displayed at various stations in London and the surrounding area.

On 6 November 2017, Class 700s started on the Great Northern route with the first, 700128, operating the 0656 Peterborough to London Kings Cross service.

[43] The Great Northern route has since been partially incorporated in the Thameslink network after through services through the Canal Tunnels began on 26 February 2018.

[48] Because the trains were to be built outside the UK, the decision to award the contract to Siemens proved controversial: there was widespread criticism of the UK government's bidding process and perceived lack of support for British manufacturing,[49][50] which in turn led to a review of governmental procurement mechanisms.

[55] In 2014, the NAO reported on the Department for Transport's handling of Intercity Express and Thameslink rolling-stock procurement projects.

The report questioned the DfT's attempt to take leadership in the project, contrary to general policy, without any prior experience of large-scale rolling stock procurement; the NAO also said the DfT had handled communications with bidders poorly, increasing the likelihood of a legal challenge to its decisions.

These poor levels of comfort, along with their tall, thin, tapered appearance, have led them to sometimes be nicknamed "ironing boards";[59] they have also been likened to sitting on concrete.

[61] Upon delivery, the trains were also missing various amenities which were considered standard, including seatback tables and Wi-Fi, which are now being retrofitted to some units.

[3] The initial livery is "light grey with pastel blue doors and a white diagonal flash at the carriage ends".

[75][76] However, in December, the Hornsey application was blocked by Secretary of State for Communities and Local Government John Denham on grounds of its scale.