The project is running on time and on budget, confirmed Crossrail, as it hosted David Cameron and London Mayor Boris Johnson at its Tottenham Court Road site in central London, 25 metres underground.

Johnson said: “There was a time when some said that building a huge tunnel deep below the streets of London was a mad plan that would do nothing for our economy.

“But as this awe-inspiring project hits its halfway point – we can see how wrong the naysayers were.

He added: “This project is not just good for London; it’s good for the whole of Britain, responsible for 55,000 jobs across the country and showing that the UK can deliver on major infrastructure projects.”

Up to 10,000 people are currently working on Crossrail, including over 280 apprentices, with the project at peak construction.

Seven 1,000 tonne tunnelling machines have bored over 29km of new 6.2m diameter tunnels under London with an eighth machine due to launch next month.

Major tunnelling for Crossrail is due to complete by the end of 2014.

A busy year underground – in pictures

Stepney Green cavern 40 metres below ground

Hanover Square

Archaeological finds at Broadgate ticket hall

Concrete segment being installed in tunnel during construction of western tunnels

Finsbury Circus SCL tunnels

Whitechapel

Finsbury Circus

Western tunnel workers

Pause for thought in western tunnel

TBM cutterhead being installed at Westbourne Park

TBM Elizabeth lowered into main shaft

Wallasea Island new nature reserve in Essex, created from tunnel spoil

Andrew Wolstenholme OBE, Crossrail Chief Executive said: “Crossrail has now reached the halfway point and is being delivered on time and on budget. We are on track to deliver a world-class railway.”