Royal Dutch Shell shed 4% to £20.57 after an oil sheen spotted near one of the firm's platforms in the central Gulf of Mexico caused the company to send a spill response vessel and seek aircraft overflights, a Shell spokeswoman said.
Traders said the kneejerk response by Shell's share price reflected concerns that any possible Gulf of Mexco spill might be compared to BP's Deepwater Horizon rig disaster in 2010.
Shell put out a statement to the London Stock Exchange this morning: "On April 11, Shell notified the National Response Center (NRC) of a light sheen in the central portion of the Gulf of Mexico, between the Mars and Ursa production area.
"Shell has no current indication that the sheen originates from wells in either the Mars or Ursa projects.
"However, out of prudent caution, Shell has activated the Louisiana Responder, a Marine Spill Response Corporation (MSRC) vessel. The Louisiana Responder is an oil spill response vessel with skimming and boom capabilities. Shell has also requested flights to monitor the one by ten-mile sheen closely with additional aerial surveillance.
"At this time, the source of this sheen is unknown, and Shell's priority is to respond proactively, safely, and in close coordination with regulatory agencies. Updates will be provided as further action is taken."
BP itself was down 1.5% at 437.85p. The firm will run the gauntlet of protests from environmentalists and investors alike at its annual shareholder meeting this morning, where it will make the latest in a series of attempts to put the Deepwater Horizon spill behind it.
"The falls by the heavyweight oils has soured the tentative recovery following Tuesday's sharp falls, although strength in the miners is helping to keep the FTSE fairly steady," said Richard Hunter, head of UK equities at Hargreaves Lansdown.
The FTSE 100 index was down around 13 points, or 0.25% at 5,621.