CCTV investigations in London Road, Kingston found a mound of fat the size of a bus had reduced the 70 x 48cm sewer to just 5% of its normal capacity.
Thames Water has begun repairs to 20 metres of the damaged pipe and work is expected to take up to six weeks to complete.
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The 34-year-old victim was left with a huge gash across her nose and face. Police need to find a man seen on CCTV.
The girl was travelling on a train from London Liverpool Street to Romford in Essex after going to see the poppies at the Tower Of London.
Wealthy businessman Ivan Massow wants to succeed Boris Johnson at City Hall in 2016.