A sewer sunk deep beneath London to stop waste from entering the River Thames has moved closer to reality

A huge new 20-mile sewer sunk deep beneath London to stop waste from entering the River Thames has moved closer to reality.

At 7m across, the Thames Tunnel would be the width of three buses.

The tunnel is intended to "significantly" reduce the amount of sewage entering the Thames by taking on overflow from the current sewer system, which dates back to Victorian times.

At the moment about 32 million cubic metres of untreated sewage - enough to fill the 02 Arena 15 times - overflows into the river every year when rain falls.

Project workers this week began exploratory works in the riverbed outside the Palace of Westminster, where a borehole was sunk to analyse the ground conditions facing engineers digging the tunnel.

The work marks the start of a nine-month programme of borehole tests along the Thames, with up to three rigs in place at a time.

If it goes ahead, the Thames Tunnel would lie below the Tube network and roughly follow the route of the river from west London to Thames Water's Beckton sewage treatment works in Newham.

The tunnel would link up with the 34 "most-polluting" points on the sewers, where waste flows into the river from overflows designed to stop sewage backing up on to the streets during freak weather.

The increased size of the city's population, heavier rainfall linked to climate change and the concreting over of green spaces have put pressure on the old sewer network and overflows tend to take place more than once a week, according to Thames Water.

Project manager Sian Thomas said: "In 1858, parliamentarians had to vacate the House of Commons and relocate to Oxford because of the foul stench coming from the nearby River Thames - then used as an open sewer.