New rules have come into force to oversee the way supermarkets treat farmers.
It came into force on Thursday (4 February) – the day before a government consultation that will decide the powers of a supermarket watchdog.
The code will be regulated by the Office of Fair Trading.
Its implementation came as further evidence emerged of retailers making retrospective demands for money and changing trading terms.
The NFU accused retailers of sustained bully-boy tactics against suppliers ahead of the code, which contains guidelines on fair dealing.
But Farmers Weekly was unable to find any producer willing to name names for fear they would be identified.
NFU president Peter Kendall said: “In the past 10 days, I have heard how suppliers to major retailers have faced some of the most unreasonable demands for retrospective payments and changes to trading terms that we’ve ever seen.”
Retailers were effectively launching pre-emptive strikes against farmers before protection afforded by the new code came into force, said Mr Kendall.
They were squeezing the very lifeblood out of British producers.
“The whole supply chain should be working together to meet the long-term challenges the food system faces; feeding a growing global population while impacting less on the environment.”
The British Retail Consortium rejected claims that supermarkets were giving farmers a raw deal.
Retailers did not control the food chain but were simply one link in a complex process from production to consumer, it said.
The implementation of a supermarket watchdog would be a step too far and prove costly for consumers, the consortium added.