The Channel Tunnel links the two countries underground and is defined as a 'land frontier', and not widely recognised as a land border.

The Treaty of Canterbury (French: Traité de Cantorbéry) was signed by British Prime Minister Margaret Thatcher and Foreign Secretary Sir Geoffrey Howe, French President François Mitterrand and Minister of Foreign Affairs Roland Dumas on 12 February 1986, and is the original document providing for the undersea tunnel between the two countries.

[2] The Treaty of Canterbury (1986) is significant and unusual because it is a modern and recent modification to the national borders of the UK and France.

[3][4][5] In the 1991 Sangatte Protocol, France signed an agreement with the United Kingdom to introduce 'juxtaposed controls' (in French, des bureaux de contrôles nationaux juxtaposés, or 'BCNJ') at Eurostar and Eurotunnel stations on immigration and customs, where investigations happen before travel.

France is part of the Schengen Agreement, which has largely abolished border checks between member nations, but the United Kingdom is not.