Anglo-Dutch oil giant Shell said Saturday it had reduced the flow from a leak at one of its North Sea platforms which has left a 31-kilometre (19-mile) long oil slick off the British coast. The leak from an undersea pipeline was discovered Wednesday after an oil sheen was spotted on the surface near the Gannet Alpha production platform, 180 kilometres (112 miles) east of Aberdeen, on the Scottish east coast. \"Shell U.K. Limited confirms the oil leak in a flow line to our Gannet Alpha platform is under control,\" Shell said in a statement. \"The subsea well was shut in on Wednesday and the flowline on the seabed is now isolated and depressurised. Leakage of oil has been considerably reduced.\" Shell said it had deployed a robot submarine to inspect and monitor the leak. A clean-up vessel and spotter plane have also been sent to the scene but the platform is still operating. \"The size of the sea surface affected is estimated to be some 31 kilometres by 4.3 kilometres at its widest point and the sheen is currently moving west from the field,\" the statement said. \"Our current expectation is it will be naturally dispersed through wave action and will not reach shore.\" It added: \"Shell takes all spills seriously, regardless of size and we have responded promptly to this incident.\" The statement did not say how much oil head leaked but a Shell source said that \"at the very most a couple of hundred tonnes\" of oil had spilt into the sea. Shell said its focus was on environmental safety and it had informed the relevant British authorities.