Animal Aid has described the cull, which is due to take place in Fife, as an insane exercise.

And it has criticised of the role of the Royal Society for the Protection of Birds, which supports the move.

The RSPB is backing the cull of the duck, which was introduced to Britain in the 1950s, because of evidence that it is now threatening the rare White-headed Duck in Spain.

The Spanish government has called for action because the Ruddy Duck has been flying over to Spain and mating with the rare bird, resulting in a hybrid.

Nationwide cull

The control measures stem from the UK Ruddy Duck Working Group, comprising representatives from government, conservation organisations, research organisations and aviculturalists.

Part of the nationwide cull will take place at Loch Kilconquhar in Fife where the birds congregate.

The loch is part owned by RSPB Scotland's chairman, the former environment minister Lord Lindsay.

He said he acknowledged that something had to be done to protect the White-headed Duck. Its population has declined to just a few thousand in recent years.

But Animal Aid said the exercise seemed designed to please a small number of "obsessives" and did not have the backing of RSPB members.

'Few' objections

The RSPB has denied the allegation, saying that only a few hundred of its million members had objected to the plan.

Ruddy Ducks are located throughout England, and into Scotland, Wales and Ireland.

In recent years, the stronghold of the UK breeding population has shifted northwards from the West Midlands into Derbyshire, Cheshire, Greater Manchester, and South and West Yorkshire.