Coastguards in Britain and Ireland were on red alert today after a Russian aircraft carrier spilt an estimated 1,000 tonnes of oil off the southern Irish coast.
The spill, which happened as the Admiral Kuznetsov aircraft carrier was refuelling at sea, caused a slick that is now more than three miles long and almost as wide.
It is the biggest oil spill in waters around the British Isles since the Sea Empress ran aground off Milford Haven in 1996, causing widespread damage to the Pembrokeshire coast.
Today's spill took place in international waters but the oil has since floated into Irish territory and is now heading for the Welsh coast where it could cause severe losses among breeding birds and marine animals including dolphins, porpoises and seals.
Surveillance flights by Britain's Maritime and Coastguard Agency (MCA) calculated that about 1,000 tonnes of oil were spilt.
Russian authorities put the figure at 300 tonnes and have yet publicly to admit their role.
The aircraft carrier, a Russian refuelling ship and a Russian tug were, however, found amid the oil when the spill was detected by a satelite on Saturday. Four other Russian naval vessels were nearby.
Samples of oil from aboard the ships have been requested by the Irish maritime authorities, who are leading the clear-up operation while the slick remains in their waters. A sample taken from the sea was on its way to Edinburgh yesterday for analysis at the MCA's laboratory.
A spokesman for the Irish coastguard said: "There was no collision. The latest we have is that it was a bunkering [fuel transfer] accident. It is military-involved so we don't have 100 per cent of the information on what happened. There is a British warship in the area and an Irish naval vessel is also on the way out."
A tug based at Cork was being loaded with specialist anti-pollution equipment today and should reach the spill tomorrow.
There was relief today among environmental groups at indications that the slick is breaking up but fears remain that some of the oil, probably formed into tar balls, will wash up on the Welsh and Irish coasts.
Mark Clark, of the MCA, said: "It's a significant oil spill and it could impact on the Irish south east coast or the Welsh coastline."
He estimated that the slick will enter British waters early next week and that it could start washing ashore in Wales in two to three weeks.
How much oil will hit coastlines remains uncertain, with weather conditions likely to play a major role, but a significant proportion is expected to wash up on rocks and beaches.
The Russian Naval Attaché in Dublin has said that an internal investigation is being carried out to determine the cause of the spill.
The ship, the only operational aircraft carrier in the Russian forces, was thought to be heading home after exercises in the Mediterranean.
At least one ocean-going tug boat routinely accompanies the carrier in case of mechanical problems at sea and on January 6 a sailor died on board the vessel when a fire broke out in an engine compartment.
Igor Dygalo, a Russian naval spokesman, would neither confirm nor deny the spill had taken place but agreed the Russian Navy "was analysing this information". He confirmed Russian warships had been carrying out refuelling operations in the area where the spill took place.
The satelite that spotted and reported the spill was operated by the European Maritime Safety Agency (EMSA) in Lisbon, a European Union specialist agency.