Its orangey-pinkish flesh glistens from countless supermarket shelves across the country but the highly prized salmon is about to undergo a chameleon-like change of colour - due to a new European Union food safety edict.

Concerned that the chemical being fed to farmed salmon to give them their bright hue may also be harming people's eyesight, the maximum amount of artificial colouring allowed in the fish by the EU is to be slashed by a factor of three.

"Brighter eyesight or brighter salmon?" was how the European commission described the stark choice yesterday.

The pigment at the centre of the scare, canthaxanthin, is also fed to chickens to give their skin and eggs a brighter yellow complexion; the maximum authorised levels for poultry will also be cut.

However, three quarters of the eggs sold in the UK do not contain the chemical at all and the levels fed to poultry are said to be well within the EU's new stricter limits.

"Scientific assessments have shown that a high intake of canthaxanthins produces an accumulation of pigments in the retina, affecting the sight," David Byrne, the EU food safety commissioner, said. "The use of this feed additive is purely cosmetic, to colour food and reduced levels of the additive will not adversely effect taste or quality."

The flesh of wild salmon is naturally pink because the fish consume large amounts of shrimps. However, almost 90% of the salmon sold in supermarkets is farmed and there is no obligation to state on labelling that canthaxanthin has been used.

Salmon farmers feed large doses of the additive to their fish because, they argue, consumers expect salmon to be pink and find the greyer shade which farmed salmon would naturally have to be a turn-off.

"It's appealing to the eye," said Julie Edgar, communications director at Scottish Quality Salmon, Scotland's main trade body. "People traditionally associate salmon with pink and red."

But using canthaxanthin carries a risk. Beate Gminde, a commission spokeswoman, said yesterday: "There's no such thing as zero risk. We do know that there's a possible impact in the long term [on eyesight] but it's impossible to quantify it."

However, she warned against alarmism. "I wouldn't hesitate to eat salmon myself."

Ms Edgar suggested that health fears about the additive were exaggerated: "You have to consume enormous quantities [of canthaxanthin] before there is even the potential for damage to your eyesight."

Food industry sources suggested that alternative methods, such as the food additive astaxanthin might be used to ensure that farmed salmon flesh stays pink.

The UK has until the beginning of 2004 to make sure salmon producers comply with the new directive.

The following correction was printed in the Guardian's Corrections and Clarifications column, Thursday January 30 2003

It was misleading to suggest in this report that in the UK the chemical is fed to poultry for meat production. Canthaxanthin is not used in rearing conventional meat chickens fed on wheat-based feed, which account for over 96% of total UK chicken, nor in free-range or organic systems.

It is relevant only to corn-fed chicken rearing and is used in some countries, but not in Britain, to enhance the natural yellow colouring which chickens acquire from corn-based feed. British producers of corn-fed chickens do not use canthaxanthin in their feed.