The design work was completed by HS2’s main works contractor, Align JV – a team of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick – working with design partners Jacobs, Rendel-Ingerop and LDA Design, and architect Grimshaw.

Two perforated concrete hoods will cover the track, extending the tunnel into the open. These ‘porous portals’ will avoid sudden changes in air pressure – and resulting noise – caused by trains entering and exiting the tunnels.

Alongside the portals, there will be a single-storey ancillary building clad in earth-coloured pigmented zinc to house mechanical and electrical equipment.

Align has now completed its entire Detailed Design including designs for all of the Key Design Elements, such as the South Portal, near the M25, and headhouses above the ventilation and emergency access shafts, which are mostly designed to resemble agricultural buildings.

Construction of the 16km twin tunnels under the Chiltern hills in Buckinghamshire recently passed the halfway point, with the two TBMs currently between Amersham and Little Missenden.

“The Chiltern Tunnel North Portal will be one of the least visible parts of the project, but today’s reveal of the final designs is a major symbolic milestone – and I’d like to congratulate Align on getting all their Key Design Elements to this final stage of development,” said HS2 design director Kay Hughes.

Align’s technical director, Alan Price, said “having the design on the shelf releases procurement and allows complete flexibility to optimise the construction programme”.

On March 9 the government announced that some construction work on HS2 would be delayed for two years in an attempt to mitigate inflationary pressure and rising project costs.

The line between Birmingham, Crewe and Manchester will be delayed by two years, meaning the service to Crewe may not be open until 2036, and Manchester not until 2043.

The government says it is committed to HS2 trains eventually terminating at Euston Station in central London, but for now it is prioritising the service between Old Oak Common in west London and Birmingham.

In 2021 the government scrapped Birmingham to Leeds section of HS2.