Exploratory works are carried out in the River Thames for a 20-mile sewer Thames Water/PA Wire

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LONDONERS may be forgiven for thinking oil had been discovered below the River Thames, if these pictures are anything to go by.

But in fact engineers were carrying out initial works on plans for an enormous 20-mile sewer to stop waste from entering the river.

If the project gets the go-ahead, the Thames Tunnel would measure 7m across - the width of three buses - and lie deep beneath the city.

It 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.

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 all 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.

"As a result, Sir Joseph Bazalgette was commissioned to build the Victorian sewer system which still serves London to this day.

"Though an engineering masterpiece, which has served the capital well, the system now needs extending to ensure it can cope with increasing sewer flows.

"Today, more than 150 years on from the Great Stink, Thames Water can be seen on the same stretch of river, planning what will be the next chapter in Bazalgette's legacy."

Together with the Lee Tunnel, a four-mile sewer to stop sewage overflows into the River Lee, the project is known as the London Tideway Tunnels.

Thames Water argued that when the regulator Ofwat sets new bill limits for water firms for the next five years, the deal must allow the company to be able to fund the work.

Peter Antolik, Thames Water's director of strategy and regulation, said: "A tough deal for Thames, which leaves us unable to fund this crucial project, will simply mean a tough deal for our customers - and, indeed, London and the River Thames."

Ofwat sets out its draft price limits later this month and the finalised limits in November.