'Helen' has begun her 3.2km tunnelling journey from Kennington to Battersea in a significant step for the Northern line extension.

The first of two giant tunnel boring machines, Helen, that will be joined in a month by her sister machine Amy, is making her way under south London to create the first of the underground tunnels that will extend the Charing Cross branch of the Northern line from Kennington to Battersea Power Station, via Nine Elms.

See related: Huge crane lowers tunnelling giants Helen and Amy 20 metres below ground in 'major milestone' for Northern Line extension

The stations will open in 2020, one at the Battersea Power Station redevelopment and another at Nine Elms that will serve developments such as the New Covent Garden Market, the US Embassy and existing communities.

One hundred metres in length, the machines were named by school children after the first British astronaut, Helen Sharman, and British aviation pioneer, Amy Johnson, the first female pilot to fly alone from Britain to Australia.

The earth- more than 300,000 tonnes- dug out will be taken up to barges on the River Thames and be transported to Goshems Farm in East Tilbury, Essex, where it will be used to create arable farmland.