Construction of the UK’s longest railway bridge has been completed on the outskirts of north-west London.

HS2 engineers installed the final deck segment of the 2.1-mile Colne Valley Viaduct on Thursday.

Around 200 contractors who worked on the project gathered at the site to celebrate the milestone.

Lowering the Colne Valley viaduct’s final deck segment into place today marks the culmination of more than 10 years of planning, design and construction

The gently curved bridge will carry high-speed trains up to 10 metres above land and water across the Colne Valley between the M25 motorway and the Buckinghamshire village of Denham.

The viaduct’s completion means the Tay Bridge – which is two miles long and links Fife and Dundee – has ceded the title of the UK’s longest railway bridge, which it held since 1887.

Work on the HS2 viaduct began in May 2022 and is being managed by contractor Align JV, which consists of infrastructure companies Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

A thousand uniquely-shaped pre-cast segments were made at an on-site purpose-built factory, and put into place by engineers using a launching girder.

The final segment features a plaque with the names of people who worked at the factory and on the bridge.

Work on the viaduct will transition to the installation of railway systems such as tracks and signalling.

HS2 Ltd senior project manager Billy Ahluwalia said: “Lowering the Colne Valley viaduct’s final deck segment into place today marks the culmination of more than 10 years of planning, design and construction.

“I pay tribute to the dedicated team that has delivered a bridge that is both the longest on HS2 and has become the United Kingdom’s longest railway bridge – taking a record that had stood for nearly 140 years.

“That is a historic achievement of which we can all be immensely proud.”

HS2 services are due to start between 2029 and 2033, running between the West Midlands and Old Oak Common, west London.