British engineers are scratching their heads wondering how to dispose of the blob, stretching 50 meters in length and weighing as much as 26 tonnes.
Although the mass of hardened grease, wet wipes, diapers and condoms is not causing any immediate issues, the sewage system built in 1852 was never designed to cope with so much waste.
Stuart White of Thames Water told AP the mass must somehow be broken up and removed before the system is completely blocked.
The find comes just two weeks after Thames Water reported another fatberg blocking a 250-yard stretch of sewer beneath the streets of Whitechapel, East London.
An eight-man team is using high-powered hoses to fragment the mass at a rate of 20 tons per day.
The fatberg is understood to be one of the biggest ever seen in the UK, measuring the equivalent length of two Wembley football pitches, and weighing the same as 11 double-decker buses.
The Museum of London has reportedly shown an interest in obtaining part of the mass to showcase it in its general collection.