Rules for labelling food's genetically modified (GM) content do not go far enough, say organic producers.
The guidelines are aimed at reducing cross-contamination
The government wants labels to show all produce with more than a 0.9% GM element, but green groups say the threshold should be nearer 0.1%.
The Conservatives back that call, saying customers need to have "clear information" to ensure trust in food.
The government says the guidelines, based on European Commission ones, are "adequate" and "appropriate".
No commercial GM cultivation is expected in the UK for several years.
The guidelines are designed to cover the possible cross-contamination of crops - for instance, from neighbouring fields - if GM crops are cultivated in future.
Under the proposed guidelines all foods - whether organically grown or otherwise - could have up to 0.9% of GM content before the GM content has to be labelled.
The Soil Association and Friends of the Earth say 0.9% is too high, saying GM contamination is measurable for an amount as low as 0.1%.
'Element of trust'
For the Conservatives, shadow environment secretary Peter Ainsworth, who hosted a meeting at the House of Commons of about 70 organic food producers and retailers, said: "It's all about the choices we make.
"The public need access to clear information when exercising choice, deciding what they buy."
Keith Abel, of organic food delivery firm Abel and Cole, said: "All of our customers are against GM full stop.
"Customers buy from a company like ours based on an element of trust.
"If up to 0.9% of the content can be GM, that trust would be undermined."
Karl Tucker, business development manager for organic dairy company Yeo Valley, said: "Customers make a choice. They don't want GM."
The EU reached general agreement in December that organic producers should work to the same 0.9% standard as non-organic producers.
A spokesman for the Department for the Environment, Food and Rural Affairs said the UK would follow this, following consultations last year.
He added: "Our proposed measures would minimise unwanted GM presence as far as possible, below the 0.9% level.
"But it is unrealistic to think in terms of a zero (or 0.1%) threshold - it would not work in practice.
"It should also be recognised that if producers had to declare any level of GM presence, no matter how small, they would be forced to label everything as 'may contain GM', which would not offer an informed choice."
The 0.9% threshold offered a "pragmatic choice for both producers and consumers", the spokesman said.