Sadiq Khan has announced tunnelling to extend London Underground's Northern line will begin in March.
A pair of two-mile long tunnels will be created to bring the Northern Line to Battersea.
The £1.2 billion project is the first major expansion of the Tube since the Jubilee line in the late 1990s.
Two new stations will be opened in 2020 - one at the heart of the Battersea Power Station redevelopment and another at Nine Elms, serving developments such as the US Embassy and the revamped New Covent Garden Market, as well as existing communities.
Two 650-tonne tunnel boring machines were unveiled today. When fully constructed below ground, they will each be 100 metres (328ft) long.