The EC Weights and Measures (Packaged Goods) Regulations 2006 permit most fruit and veg to be sold in average pack sizes that carry the e-mark, which proves a line satisfies the EU's rules for average weight packs.
However, an annex to the regulations excludes 36 crops from the regulations.
Produce on this list include common crops such as grapes, tomatoes, carrots and onions.
Fresh produce bosses are baffled because the regulation does not explain why these foods have been excluded - nor what measures should apply to them instead. The only thing the designated crops appear to have in common is that they are covered by a European Union marketing standard.
The inclusion of the annex means that since April 6 it has been illegal to pack these crops in average weight packs.
The DTI is pressing for an amendment to the regulations, which were designed to simplify previous rules, but it is not known how long this process will take. Any alteration could be as simple as deleting the annex from the regulation, said Douglas Pattie of the Fresh Produce Consortium. He added: "As some point there must have been a reason for creating the annex, but this has been lost along the way."
There is concern that over-zealous Trading Standards officers could take action against retailers continuing to sell products listed in the annex in average weight packs, though Lacors said it would urge local authorities to take a common-sense approach.
Some retailers said they were already aware of the situation. A Morrisons spokeswoman said: "We are seeking clarity from Trading Standards as to the interpretation and implications."