Video images from a news helicopter had shown a patch of flames on a burst pipe on a tower at the Coryton refinery 25 miles (40 kilometers) east of central London, near the mouth of the Thames River.
"Crews on the scene report cooling operations going well and estimate no further risk of development," a regional fire department reported.
Firefighters were checking whether the tower was structurally stable, and deciding whether to remove its contents.
Seventeen fire engines responded to battle flames reportedly rising to 100 feet (30 meters).
BP PLC sold the refinery in May to Swiss-based Petroplus Holdings AG.
The fire department said everyone at the facility had been accounted for, and homes in the vicinity were not evacuated.