Whitehall mandarins have unknowingly been sitting on a giant “fatberg” of congealed slime embedded with condoms, sanitary products and faeces.
Technicians from Thames Water have been sent in to clear the colossal blockage in a sewer beneath some of London’s most prestigious streets.
It was caused by fat clinging to wet wipes in the two-metre high tunnel, eventually congealing to form a foul-smelling solid mass.
A painstaking operation was needed to clear it because of its historical Westminster location, using specialist equipment to suck away the fat and high-powered jets to clean the walls.
Dave Dennis, the sewer manager for west London, said: “We find objects down the sewers every day that should not be there. Planks of wood, plastic bottles, and the bane of our lives – wet wipes.
“They may say flushable on the packet but they don’t break down inside the sewer. They cling to cooled cooking fat and form fatbergs, which block pipes and flood our customers with sewage.”
He described the mass "like concrete" to an ITV reporter who went into the tunnel.
Thames Water spends approximately £1 million a month clearing fatbergs and blockages, dealing with 55,000 a year.
Mr Dennis pleaded with people around the country to avoid the “horrific experience” of sewer flooding by stopping washing fat down drains.
“Please remember next time you go to throw cooking oil down the drain or wet wipes down the loo -bin it, don’t block it,” he said.
Earlier this year, another fatberg the size of a Boeing 747 was discovered beneath the streets of west London, in Shepherds Bush, and a record-breaking blockage in Kingston weighed 15 tonnes.