The two 2,000t TBMs will be launched early next year to dig the 10-mile-long Chiltern tunnels – the longest bore on the project.

At 136 acres – the size of 80 football pitches – the South Portal Chalfont Lane site will be the biggest construction site playing a key role in phase one.

From here, specialist teams will deliver both the tunnelling and the construction of the adjacent Colne Valley Viaduct.

Work is now also underway to build a concrete precast plant where more than 118,000 tunnel segments will be made.

The South Portal Site is the HQ for main works contractor, Align JV.

Around 350 staff from Align JV – a team made up of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick, and its subcontractors – are now permanently based on site.

At peak, around 1,200 people are expected to be employed in the design and construction of the Chiltern tunnels and the viaduct, with 50 opportunities for apprentices.

The ‘twin bore’ Chiltern tunnels will not only be the longest but also the deepest tunnel bores on the route, with separate northbound and southbound tunnels and five ventilation shafts.

Just south of the tunnel portal, the Colne Valley Viaduct – designed by Jacobs and Ingerop-Rendel, and architect Grimshaw – will be the longest railway viaduct in the UK.

At 3.4km long, it will carry HS2 across the Grand Union Canal and surrounding lakes.

Further south, the line will go into tunnel again before arriving at Old Oak Common and London Euston.

HS2 delivery director, David Bennett said: “The south portal site will have one of the most important parts to play in the story of HS2. From here we’ll manage the construction of the Chilterns tunnel and the iconic Colne Valley viaduct.

“It’s great to see how much progress has been made on site – and I look forward to working with the team to deliver what will be two of the most exciting and ambitious engineering challenges on the project.”

Daniel Altier Align JV project director said: “Significant planning and preparation has been completed. We now have a significant presence on-site and this is set to increase over the weeks and months ahead.

“Align is now actively recruiting to build our team to address the complex engineering challenges associated with the delivery of this iconic project.”