HS2 has started construction of the UK’s longest railway bridge.

The Colne Valley Viaduct near Rickmansworth will stretch for 2.1 miles (3.4km), carrying high-speed trains some 33ft (10m) above a series of lakes and waterways.

A 160-metre long bridge building machine known as a “launching girder” is being used to lift the giant concrete segments that form the viaduct’s arches into position.

The 1,000 segments each weigh up to 140 tonnes and are being made at the HS2 compound in Maple Cross.

Production of the segments began in February and is expected to take just under three years, with the viaduct completed by summer 2025.

HS2 minister Andrew Stephenson said: “Today, HS2 began construction on what’s set to be Britain’s longest railway viaduct, a landmark moment for HS2 and a feat of British engineering, taking the HS2 line from London and into Hertfordshire and Buckinghamshire.

“Infrastructure is the backbone of HS2 and this viaduct will be integral to delivering faster journeys and an increased capacity rail network.”

HS2 Ltd chief executive Mark Thurston said: “I’m absolutely delighted that we have started work to assemble the giant deck segments that will form the Colne Valley Viaduct.

“It is yet another big milestone for HS2 Ltd, as we work to deliver the UK’s new high-speed railway.

“Once complete, this record-breaking structure will form a key part of the HS2 railway – helping to deliver better connections across the UK, free up rail capacity on the train network, and offer passengers a zero-carbon travel option.

“I’d like to thank all those involved in getting us to this exciting stage and look forward to seeing the whole viaduct come together over the coming years.”

The launching girder is the only machine of its kind in the UK.

It was built in 2004, and was first used in Hong Kong.

Designers of the viaduct were tasked with minimising its impact on the environment.

It is being set low into the landscape, taking inspiration from the flight of a stone skipping across water.

The work is part of a £1.6 billion contract handed to joint venture Align, which also involves building a 10-mile (16km) tunnel under the Chiltern Hills.