Otters may leave the Thetford area if pollutants - such as as diesel and engine oil - continue to end up in the waterways, an animal welfare officer has warned.

Craig Plumley, an animal welfare officer for the RSPCA, spoke of his concern for the area’s otter population following a diesel spill just over a week ago which saw about 4,000 litres of fuel leak into the River Thet.

As a result of the spill, which is thought to have happened when oil thieves were disturbed by security guards, about 25 river birds were rescued by RSPCA officers, with help from the British Trust for Ornithology (BTO), but three signets and a Canadian goose have died.

Mr Plumley said this was the third case in the past year of oil contaminants entering the water at Thetford, with the previous two involving engine oil.

While no dead or sick otters have been discovered following the diesel spill, he said if these incidents continued “we run the risk of losing them”.

“Otters don’t stay around in polluted water. We have only just got them back; we don’t want to lose them because of stupidity.”

Otter numbers in England and Wales declined dramatically from the mid 1950s to the late 1970s, but their return to most of England is considered by environmentalists as one of the major conservation success stories of the last 30 years.

Mr Plumley said photographers came to Thetford in the hope of capturing an otter.

“It’s down to other agencies to make sure these pollutants don’t get into our waterways and we will work if necessary to try and help stop this in the future. Three [incidents] in a year is a bit worrying for the wildlife.”

He added: “I don’t think people realise what they put into the drain ends up in the river, but it does.”

He said it was a “huge testament” to the quick work of the BTO, Environment Agency (EA), Anglian Water and the RSPCA that the damage to the birdlife following last week’s spill was not “catastrophic”.

A spokesman for the EA said booms had contained the spill and absorbent booms have helped the river to improve.

He added: “There’s no evidence of distress or death to the fish, but we are still investigating that.”

The rescued birds were taken to the RSPCA’s Norfolk Wildlife Hospital at East Winch, near King’s Lynn.

Mr Plumley, who said diesel clears quicker than a lot of other oils, believes the bird will be reintroduced into the wild in the next couple of weeks.