Sewage overflowing onto a grass verge outside the BP garage in Easebourne Lane was due to a blocked toilet pipe.
The overspilling manhole cover was reported to the Environment Agency yesterday by a concerned resident.
This afternoon, the Environment Agency said the blockage had been cleared.
A spokesman said: “We contacted Southern Water to carry out an investigation.
“We received an update this morning confirming Southern Water had found the source on land owned by BP, it was confirmed as a discharging manhole due to BP’s toilets foul pipe being blocked.
“Southern Water confirmed it was a spill contained on land around the manhole and there was no evidence of a pollution to the surface water drains and no impact on water quality.
“BP employed a drainage contractor to unblock the foul pipe and the incident has been resolved.”
A spokesman for Southern Water said: “ We’ve carried out checks on our own assets and believe everything is working as it should.
“We found a blocked manhole inside the property to be causing the flooding and a private company is assisting the owner.
“We know how distressing internal flooding can be and we will always investigate the causes and advise or assist customers.”