The £14.8 billion east-west railway tunnels are part of the Crossrail programme.
The Government project aims to spend over £38 billion between 2014-19 on improving and maintaining the UK's railways.
The tunnelling project began in summer 2012, with eight 1,000 tonne machines boring 42km of 6.2m diameter rail tunnels under the capital.
The next phase of the programme will focus on fitting out the tunnels and developing new stations.
-- Advertisement --
Already 13.5km of new passenger, platform and service tunnels have been developed using a sprayed concrete lining technique.
London Mayor Boris Johnson said: "This is a landmark moment for London that puts us a gigantic step closer to the launch of an absolutely vital new railway, which will hugely improve our ability to speedily move people across our city."
The Chairman of Crossrail, Terry Morgan, said: "The completion of Crossrail tunnelling is a truly significant milestone and would not have been possible without the support and commitment of London, our contractors and everyone who works on Crossrail."
Transport Secretary Patrick McLoughlin said: "This is a major milestone in the most ambitious rail project this country has seen for decades."
(LM)
