Royal Dutch Shell said it had moved closer to stemming Britain's biggest oil spill for over a decade by closing a valve from which crude had been seeping for over 8 days.
"Closing the valve is a key step," said Glen Cayley, Technical Director of Shell's exploration and production activities in Europe.
"We will be watching the line closely over the next 24 hours and beyond," he added.
Divers closed the relief valve on Friday, after being sent down to inspect the faulty valve at the Gannet field 180 km off the Scottish coast. Shell said it had "made good progress" in stopping the leak from a flowline to the Gannet Alpha oil platform.
"Our next task is to remove the residual oil from inside the depressurized flowline, and that will take time," added Cayley.
The rate of the leak had slowed to less than one barrel a day, and approximately 218 tonnes of oil had entered the sea since the problem was detected on Aug 10, Shell said in a statement on Friday.
Hugh Shaw, appointed by the UK Government to oversee the operation, said aerial and subsea surveillance would show whether this had worked.
"Shell informed me at 10.58 (0958 GMT) this morning that both valves have been closed by divers, though I must be clear that this is not the end of this particular phase of the operation as there will now be a period of extensive monitoring to determine whether the operation has been successful and whether the leak has been stemmed," said Shaw.
Shell, which co-owns the Gannet field with US oil major Exxon Mobil Corp , said an area on the sea surface of around 6.7 square kilometres was currently affected by the oil spill, according to Marine Coastguard's estimates.
Shares in Shell were down 2.41% at 1620 GMT, lagging behind the European index of oil and gas companies which was down 1.71%.