Scientists investigating mass pollution affecting scores of sea birds and thought to have killed hundreds more say the source of the contamination may never be known.
Around 300 birds, mostly guillemots, were treated at the RSPCA West Hatch centre near Taunton, Somerset, over the last three weeks.
Plymouth University had confirmed the substance was likely to be polyisobutene, but the Maritime and Coastguard Agency said it had been unable to trace the source.
