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The tunnelling work on the £1.2 billion Northern Line extension to Battersea Power Station will begin in March, it was announced today.

Mayor Sadiq Khan confirmed the start date for the first major addition to the Tube map since the late Nineties as two gigantic tunnel boring machines - named Helen and Amy - were unveiled in Battersea.

The spur from the Charing Cross branch of the Northern Line will start and Kennington and end at Battersea, with two stations at Nine Elms and the power station.

The two machines were built by NFM Technologies in Le Creusot in central France.

The 650-tonne machines are undergoing final testing before commencing work on the two mile long eastbound and westbound tunnels. They are named after the first British astronaut, Helen Sharman, and British aviation pioneer Amy Johnson, the first female pilot to fly solo from Britain to Australia.

Each one is capable of tunnelling up to 30 metres per day with teams of around 50 people needed to operate them.

The work is expected to take around six months to complete.

As they advance forward, nearly 20,000 pre-cast concrete segments will be built in rings behind them.

Mr Khan said: “It’s great news that we are going to begin tunnelling for the Northern Line extension. Extending the line to Nine Elms and Battersea is going to be a real boost to south London, with the improved transport link helping to provide thousands of homes and jobs for Londoners. I’m also delighted that local schoolchildren have chosen two such inspirational British women as the names for these tunnelling machines.”

The machines will tunnel at depths of 26 metres for six months, excavating more than 300,000 tonnes of earth. This will then be passed along conveyors before being loaded on to barges and taken to Goshems Farm in East Tilbury, Essex, where it will be used to create arable farmland.

Mark Wild, managing director of London Underground, said: “This is a historic moment for London Underground as we prepare to start tunnelling to create the first extension to our iconic Tube network for nearly two decades. The Northern Line Extension will bring Battersea and surrounding areas to within 15 minutes of the West End and City.”