Angry growers in Kent prompted the NFU to question the retailers and marketing desks about the volumes of English plums being sold and current promotional activities.
Kent grower Nigel Bardsley told the union that the British plum market was in a critical state. He said growers were leaving their crops to rot and some were considering uprooting their trees.
"Imported produce is being dumped on our market and sold at less than the cost of production," he claimed.
An NFU South East spokeswoman said: "The English plum season started early, but the Victoria plum is very hard to find on supermarket shelves. The NFU wants to know why, when we know consumers really want to buy English plums."
NFU head of food chain Lee Woodger said: "The UK has seen a bumper crop this year and supplies of British plums are plentiful. However, it seems that a large proportion of plums on supermarket shelves are imported.
"Retailers need to be aware that while it might be tempting to stock lower-quality imports this year, they will not always be available. They should work with UK growers to ensure the industry remains sustainable and can maintain a secure supply."
He continued: "The big four supermarkets need to do more. Despite the bumper crop, up to three-quarters of the plums on shelves are imported.
"The supermarkets claim they are doing a great deal to sell, as they call it, the glut. This isn't borne out."
In some cases they were "misleading" shoppers with signage saying English produce was bang on season, only to offer imported plums on the shelf, he added. The NFU was trying to identify alternative markets such as London wholesalers, he said.
Robert Pascall, who grows plums in Maidstone, Kent, and who voiced concern two years ago over the same issue, said: "The trouble is we don't have a dedicated crop association for plums and there's no communication between producers at all.
"We are all fighting our own corners, which is why we are in the position we are and exposed to such difficulty. Two years ago we were stuffed because multiples did not allocate shelf space."
A Sainsbury's spokesman responded: "The reports are not true - we have seven varieties and give English plums a lot of support. Our growers are favourable of what we are doing."
Tesco directed Grower to the British Retail Consortium, where spokeswoman Sarah Cordey said: "It would be ridiculous for retailers not to be selling British plums. They are promoting them with bigger punnets and marketing drives. There's been a glut and retailers are doing all they can to promote demand, but there is a limit to what they can do."
Any growers who would like an update on the situation can email Woodger at email@example.com.
"The situation is absolutely unsatisfactory. There is quite clearly a huge demand by consumers. But we need greater communication between the different marketing desks. Unless marketing groups can indicate they are taking English plums seriously as an important part of their business and they can do the job of marketing, growers should say 'no, I will get my produce sold by someone who can do the job'."
Adrian Barlow, chief executive, English Apples & Pears.