HS2: Work begins on 'UK's longest railway bridge' in Buckinghamshire

Work has begun on the HS2 project's Colne Valley Viaduct which is designed to be the UK's longest railway bridge.

The structure, at 3.4km (2.1 miles) in length, will carry the high-speed rail line over the Colne Valley Park near Denham in Buckinghamshire.

A 700-tonne machine will lift 1,000 concrete segments for the arched bridge into place - each the size of a double decker bus.

HS2 Ltd said it was a "big milestone" for the project.

The HS2 project is creating high-speed rail links between London and cities in the West Midlands and North of England.

The line's first viaduct heading out of the capital will stretch across lakes and waterways between Hillingdon and the M25 and continue northwards between Chalfont St Peter in Buckinghamshire and Chorleywood in Hertfordshire.

A series of spans - sections of bridges between supports - some up to 80m (262ft) long, will carry the railway about 10m (33ft) above the surface of the lakes, River Colne and Grand Union Canal.

The 160m (525ft) "launching girder" machine, which has previously been used to build bridges in Hong Kong and Singapore, has been shipped across the world to help assemble the concrete deck segments - each one weighing up to 140 tonnes.

To allow for the curves of the viaduct as it crosses the valley, all the segments are slightly different shapes and made on site at a purpose-built temporary factory nearby, which is larger than the Royal Albert Hall.

HS2 minister Andrew Stephenson MP said the start of construction was a "landmark moment for HS2 and a feat of British engineering".

"Infrastructure is the backbone of HS2 and this viaduct will be integral to delivering faster journeys and an increased capacity rail network," he said.

Mark Thurston, HS2 Ltd's chief executive, said: "It is yet another big milestone for HS2 Ltd.

"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."

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