Contractors building a new viaduct for HS2 have made history as they lowered into place its final deck segment in an operation that crowns it as Britain’s new longest rail bridge.

The 3.4km(2.1 mile) long structure on London’s north-western edge stretches across the Colne Valley near the M25 motorway and the village of Denham.

It will carry high speed trains running to and from the capital at speeds of up to 320km/h (200mph).

The viaduct’s completion means it is slightly longer than the 3.3km Tay Bridge linking Fife and Dundee which had held the record as Britain’s longest rail bridge 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.

It is one of 500 bridging structures on the HS2 project which also include footbridges, drainage culverts and innovative 'green bridges' for wildlife.

Building work on the viaduct’s deck began in May 2022.

Over the following 28 months a team of engineers used a massive launching girder to lower into place the bridge’s 1,000 uniquely-shaped deck segments.

Edging forward from north to south and supported on the viaduct’s 56 piers, the 160 metre 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 1,000 pre-cast segments are made on-site at a purpose-built factory. Each of the segments is uniquely shaped to enable the structure to curve as it carries the high speed line up to 10 metres above land and water across the Colne Valley.

Once the main civil engineering phase of construction ends, the factory and surrounding buildings will be removed and the whole area between the viaduct and HS2’s 10-mile tunnel beneath the Chiltern Hills just to the north will be transformed into an area of chalk grassland and woodland as part of HS2’s ‘green corridor’ project.

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 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.”

The milestone comes a week after the completion of HS2's first major viaduct, at Highfurlong Brook in Northamptonshire.

Work is also progressing on the Delta Junction viaducts in the West Midlands and the elevated approaches to Curzon Street station.