The Food Standards Agency (FSA) said there was not enough "quality evidence" to make a judgement on the effect of diet and nutrition on children's learning, education and school performance.
The agency commissioned researchers at the University of Teeside to assess a range of studies covering breakfast, short-term sugar intake, fish oil supplements and vitamin and mineral supplementation.
Its conclusion will be disappointing to the food industry and those advocating healthier school meals. It also delivered a particular blow to Dairy Crest's St Ivel Advance omega-3 milk.
"There is insufficient evidence to reach a firm conclusion on the effect of omega-3 fatty acids on the education or learning of the general population," said the FSA.
The comment came only two weeks after Britain's Advertising Standards Authority (ASA) told Dairy Crest to remove St Ivel Advance milk adverts that claimed the product could enhance children's ability to concentrate and learn.
Both authorities' stance conflicted with that of high profile scientist Lord Robert Winston, who appeared in the Dairy Crest ads.
"I have carefully reviewed the full text of well over 200 scientific publications, published in peer-reviewed journals, which address the possible merits and dosage of omega-3 in adults and children in various situations and conditions," said Winston.
"There is a very strong case for encouraging parents to give their children more Omega 3," he added.
The ASA and FSA noted that omega-3 supplements had appeared to help children with learning difficulties in the Oxford-Durham Study, which used supplements made by Equazen.
But, the FSA said it wanted to see more long-term research carried out into the effects of diet and nutrition generally on school children's ability to learn.
The agency maintained its current guidance that eating breakfast and a healthy, balanced diet was beneficial for a child's development and general health.