The Office of Fair Trading has had to delay its decision on whether to refer the supermarkets to competition watchdogs because of a flood of extra evidence received at the last minute.
The OFT announced last month that it was minded to ask the Competition Commission to examine the grocery market, amid complaints that the big retailers were abusing their power.
But the decision was sent out for industry consultation along with an invitation for further evidence.
Previous attempts to show that supermarkets are unfairly squeezing suppliers and bulldozing high-street competition have foundered because of a lack of proof.
But suppliers seem at last to have stepped out of the shadows, with more than 1000 submissions made over the last month.
A spokesman at the OFT said it would probably take until mid-May to read all the new letters.
A decision on the referral is expected soon afterwards.
The NFU sent in its own submission in March, calling for the widest possible inquiry.
President Peter Kendall said: “Much of the competition between the retailers seems to be based solely on who can sell food at the lowest price.
“I believe this is an incredibly short-term approach with the potential to disastrously affect the long-term viability of producers and eventually limit the range of products available to consumers.
“As professional food producers, our objective is to deliver high-quality, safe food for consumers at realistic prices.
To do this, we need open and transparent trading arrangements across the supply chain.”