The gently curved 3.4km-long viaduct will carry high-speed trains up to 10m above land and water across the Colne Valley between HS2’s London tunnels in Hillingdon to the Chiltern tunnels.

The viaduct’s completion means that the 3.3km Tay Bridge linking Fife and Dundee has finally ceded the crown of Britain’s longest rail bridge to HS2, a record it had held since 1887.

The Colne Valley viaduct’s construction is being managed by the main works contractor Align JV – a team consisting of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

Building work on the viaduct’s deck began in May 2022. Over 28 months engineers used a massive launching girder to lower into place the bridge’s 1,000 pre-cast deck segments, each unique to create the curve.

Edging forward from north to south and supported on the viaduct’s 56 piers, the 160m long launcher used a balanced-cantilever method to lower deck segments into place to form half an arch either side of a pier before moving to the next pier to complete the arch span by repeating the process.

The viaduct’s construction will next move into the rail systems installation phase as it progresses towards becoming an operational part of the new high speed railway between 2029 and 2033.

HS2 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.”