Sewage poured into the basement of an accountancy firm after torrential rain caused a drain to burst.
Employees at Hammonds Accountants, in Pinner Road, North Harrow, were left ankle deep in sludge after the pipe overflowed and exploded on Saturday.
Director Dipak Shah rushed to work could only watch in horror as mud spewed from the firm’s car park into the basement of the building.
The access road behind the building has been left covered in mud, and rats have begun to infest the area.
He said: “It was sickening and disgusting. There were six to eight inches of muddy water in the basement.
“Puddles of sludge and sewage water were pouring in but there was nothing we could do about it. It all smelt absolutely terrible.
“Everything is still soaking wet. It’s a health hazard and I feel it’s too dangerous for us to clear up ourselves - especially since we’ve had rats here.”
The 51-year-old said the area is often blighted by rats as the car park is used as a fly tipping spot - but the flood has exacerbated the problem.
He has contacted both Harrow Borough Council and Thames Water, but claims each have denied responsibility for owning the drains.
During working hours, he is forced to keep all the doors and windows open in an attempt to ensure the water dries out.
He added: “It’s a demoralising sight to come into work and see your office like that. I just want it sorted out.”
Since November 2011, any drain serving two or more premises is the responsibility of the water company.
But if the drain only serves one property, it is up to the building owner or the tenant to clear any blockages.
Mr Dipak believes over 15 properties behind the access road use the drain as a nearby pub was also left severely flooded.
A statement from Thames Water said: “We know many of our customers are having a difficult time dealing with flooding.
“We’re under enormous strain too – with a network that’s only supposed to take waste water being inundated with floodwater, we’re working extremely hard to manage sewer levels.
“Teams are working around the clock, our resources are stretched and we’re on constant high alert, but we’re doing our best in difficult circumstances.”