Thames Water says it “let customers down” after 1,000 properties flooded across London last summer.
The water company has apologised to Londoners whose homes and businesses were severely flooded during intense storms on 12 July and 25 July.
An internal review found some residents had raw sewage spilling into their homes.
Thames Water CEO Sarah Bentley said she was “deeply sorry” for how the firm dealt with the aftermath of the storms.
About 1,000 buildings including homes, schools, businesses, hospitals and Tube stations were flooded.
Most of the properties flooded on 12 July, when a month’s worth of rain fell in an hour.
The Environment Agency issued two flood warnings for areas in London near Beverley Brook in Worcester Park and West Barnes.
Roads in the capital were closed because of the flooding, including the Blackwall Tunnel, the A12 and parts of the North Circular.
Eight Tube stations and one Overground station were also closed.
Sewers flooded in west London as heavy rainfall combined with high tides, which prevented drainage systems overflowing into the Thames.
The report said Thames Water was slow to respond to the floods because it had struggled to understand what caused them and their impact.
It also said the company did not plan for the storms properly, gave customers unclear messages about what to do, and failed to update local councillors and MPs about what was happening.
Reacting to the report, Mrs Bentley said: “We did not have the capability in place to provide a better response for our customers.
“The flooding events which took place in July show how much we need to do to turn things around for the better and we know that our immediate priority is to focus on getting the basics right.”