A flaw in the revised Groceries Supply Code of Practice renders it unenforceable, a leading competition lawyer has claimed.

Retailers would not be legally obliged to comply, warned Michael Hutchings, who was heavily involved in the groceries market inquiry on behalf of supplier organisations. The order bringing the code into law next month would only require retailers to include the code in their contract with suppliers, he said.

It did not impose a direct legal obligation to follow the code. This meant suppliers would have to accuse retailers of breach of contract - which many would not do because of the potential repercussions.

"This ignores the representations made to the Competition Commission that suppliers are not going to risk making complaints to their supermarket customers for fear of retribution," said Hutchings. "It is as if the commission has gone 99% of the way on the code of practice, but the last 1% that actually makes it work is missing."

For the code to be effective its terms had to be included in the order, he said.

A commission spokesman said this was not legally possible, but denied it rendered the code ineffective. "The code, by being included in contracts, will provide greater clarity and protection for suppliers," he said. "It is there to prevent disputes, but if they do arise, then ultimately there's an onus on people to use their rights."

The new code prohibits retrospective changes to terms and limits the extent to which suppliers must pay for listings, promotions, inaccurate forecasts or customer complaints.