It carries 125-miles of cables beneath the streets and will supply 20 per cent of London's electricity needs
The London Power Tunnels, opening this month, cost £1bn to build over seven years. It will now supply 20 percent of the capital's electricity needs.
The Tunnels were given the royal seal of approval yesterday by the Prince of Wales and The Duchess of Cornwall, who visited Highbury substation in north London.
The National Grid project becomes the most significant addition to London's electricity system since the 1960s.
The tunnels run from Hackney in the east to Willesden in the west, and from Kensal Green to Wimbledon in the south.
During construction, workers made use of electric cars above ground and bicycles in the tunnels below to reduce dangerous emissions.
Donning a hard hat emblazoned with the words National Grid, and a bright orange high-visibility jacket, the Prince looked in good spirits