Two perforated concrete hoods will cover the track, extending the 10 mile long tunnel into the open air.

These ‘porous portals’ will avoid sudden changes in air pressure – and resulting noise – caused by trains entering and exiting the tunnels.

Set low into the landscape between Great Missenden and South Heath in Buckinghamshire, the portals will only be visible from a footbridge over the railway to the north.

To allow for the different levels of air pressure, the portal for trains entering the tunnel will be 220m long, while the one for those exiting will be just 135m. Both will have smooth concrete on top and textured concrete to a low level, to reduce staining and maintenance.

The structures have been designed and will be built by HS2 Ltd’s main works contractor Align JV – a team made up of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

Work is well underway, with two enormous 2,000 tonnes tunnel boring machines heading north from the south portal to excavate the twin-bore tunnels. They reached Chalfont St Peter last week and are expected to breakthrough at the north portal in two years’ time.

Alan Price, Align JV Design Director said: “Our TBMs, Florence and Cecilia are making good progress and have each completed over 3km on their journey to the north portal.

“We are always very conscious of the sensitive setting for any permanent structures on the project and this includes the north portal design. We welcome comments from local stakeholders in order that we can finalise the design.”

The surface route to the north of the tunnels, is being built by another of HS2’s main works contracts, EKFB – a team made up of Eiffage, Kier, Ferrovial Construction and Bam Nuttall.