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Homes near Google’s £1 billion London headquarters enjoy Britain’s fastest internet while swathes of the capital suffer some of the worst speeds in the country, a new interactive map has revealed.

The web giant’s new King’s Cross offices are plumbed with blisteringly fast gigabit ethernet and wifi able to download a two-hour high definition movie in 25 seconds, and it seems homes nearby are benefiting.

But while the newly-built N1C is Britain’s fastest postcode, people living near the Olympic Park have among the slowest download speeds, despite the billions spent on 2012 Games infrastructure.

The capital’s broadband gap was revealed for the Consumer Data Research Centre by University College London maps expert Oliver O’Brien, who used average residential download speeds from regulator Ofcom.

Mr O’Brien’s searchable map shows outer London and the suburbs generally enjoying much faster internet than those in packed urban areas.

Low speeds are shown on the map and light and dark pinks, notably Westminster and homes in the Square Mile, whereas City firms buy in their own costly high-speed ethernet.

Rotherhithe has some of London’s worst speeds, with many neighbourhoods suffering download speeds of under 5mbps.

Infrastructure problems there include ageing copper telephone wires failing to support high-bandwidth demands.

One resident said his connection is usually about 0.26 Mbps, when even a 1mbps connection would take six hours to download an HD movie.

Local residents report going to the local library or a café to send emails with attachments.

But at another bend in the Thames, at Barnes, people online enjoy connections of well over more than 70mbps, the highest bracket on Mr O’Brien’s map, show in dark green.

Much of Islington is also dark green, while Enfield, Twickenham, Redbridge and Barking & Dagenham also enjoy some of London’s fastest fibreoptic speeds.

Broadband provider BT Openreach said many Londoners could upgrade to faster connections if desired, but were choosing slower, cheaper packages.

Release of the map follows the publication last week of the London Assembly report that ranked London 30th out of 63 British cities for internet speed and was found to have failed to invest in ultrafast fibre optic cabling.

The Digital Connectivity in London report said the capital lagged behind York, Coventry and Edinburgh, which enjoy gigabit speeds.

Mr O’Brien, a senior research associate at UCL’s department of geography, digital cartography and data visualisation, said: “The very centre of cities often show slower speeds than the suburbs.

“This is possibly because of the difficulty of installing the needed infrastructure under narrow, busy streets and through old, often historic buildings.

“By contrast, newer housing developments, normally on the edge of cities may come with broadband designed into the plans.”

An Openreach spokesman said: “Independent data shows that almost 96 per cent of homes and businesses in London can order a superfast service of 30Mbps or more today – and any business can order a dedicated ultrafast connection.

“But there’s more to do, and that’s why we’re investing billions of pounds across the country to extend our network and make faster speeds available to more people.”

CLICK HERE TO SEARCH THE MAP