ENVIRONMENT chiefs have launched an investigation after 35 swans on the River Itchen were left coated in oil a week after an identical spill.

Animal welfare officers in Southampton have been forced to move an entire flock of 40 swans away from the affected stretch of the waterway until the source of the slick is found.

They are promising to pursue those responsible for the spill through the courts. They think the source could be a nearby housing estate.

The latest oil slick was discovered yesterday - exactly seven days after the Environment Agency and the RSPCA were first called to Riverside Park in the Bitterne Park area of Southampton.

Then 18 swans were found covered in black oil, along with two grebes that had died from exposure, though at first the authorities could not work out where the substance came from.

The slick eventually dissipated and as the affected swans had all been caught and taken to animal sanctuaries for cleaning, no further action was taken.

Yesterday staff from both agencies were again called to the site and found double the number of swans plastered in oil.

A spokesman for the Environment Agency said: "The problem is that it's on the surface of the water, so as the swans swim through it or poke their heads through to feed, they get covered in the stuff.

"Once it's on them they are in trouble because they can't get it off and it affects their insulation, buoyancy and ability to feed. In short it kills them."