Manufacturers say they will reconsider their commitment to Guideline Daily Amounts as the best form of front-of-pack signpost labelling if a major survey by the Food Standards Agency provides evidence the scheme doesn't work.

The research, set to begin this year, will assess consumers' understanding and the effectiveness of the GDA scheme, the FSA's multiple traffic-light labels and a hybrid of the two.

The FSA has already pledged to stand by the results of the study in terms of its approach to signpost labelling. And this week the GDA Campaign, backed by the Food and Drink Federation, indicated it too could be swayed by the outcome. "If there is conclusive evidence GDAs don't work then we would definitely think again," said campaign director Jane Holdsworth. "But every metric used so far has proved GDAs are working very positively. Our most recent research found 80% of people were aware of GDAs, 54% used them and 84% thought they were easy to understand."

There were now eight retailers, 40 food manufacturers and two foodservice operators using pure GDAs, she said. Yesterday (Friday), all 50 companies were invited to the campaign's first GDA Adopters Forum at which Holdsworth urged them to ensure they were all dispensing consistent advice to consumers.

Holdsworth also told them the use of GDAs was becoming widespread on the Continent, through the likes of Kellogg's, Coca-Cola, PepsiCo and Danone.

However, Europe also brings a cloud on the horizon for supporters of GDAs. Under a review of EU labelling laws, Brussels is considering basic principles that member states would have to apply to labelling.

No proposal has been produced yet, but one could be the portion size on which schemes are based. If the EU adopts a 100g portion as the standard portion - as the FSA has for traffic lights - it could be bad news for GDAs, which are based on variable portion sizes.