British farms had 4,300 unfilled vacancies in 2017 with food left to rot in fields and farmers facing significant financial losses, according to survey data from the National Farmers Union (NFU).
The survey shows that 99% of seasonal workers who were recruited came from Eastern Europe, with just 0.6% from the UK.
The NFU's data showed that 12.5% of all vacancies were unfilled in 2017 - the first time it has recorded a shortfall since the survey began in 2014. In the harvest month of September - crucial for the fruit and vegetable industry - 29% of vacancies were unfilled.
The proportion of previous workers returning to the UK has also fallen, from 41% in 2016 to 29% in 2017.
"Growers are wondering how they are going to get through the  season," said Alison Capper, chair of the NFU's horticulture board. "There is an element of desperation."
Capper said that farmers had been forced to leave food to rot in fields.
Farmers have been warning of the dangers of agricultural labour shortages since the UK voted to leave the EU in 2016. Agricultural organisations have called on the government to reintroduce the seasonal agricultural workers scheme (Saws) - which closed in 2013 after 60 years - to address the shortage.
Documents released to the Guardian under a freedom of information request show that the government's Saw Transition Working Group acknowledged that farms were "struggling" in July 2017.
The group discussed "creative options" to fix the problem, including "working holiday" schemes similar to those used in Australia and New Zealand.
Capper told the Guardian: "[Australia and New Zealand] do have a working holiday visa scheme, but the vast majority of their fresh produce is picked by people on a seasonal workers scheme from Tonga and Samoa, the same way as our old Saws."
The Labour Party has committed to reinstating Saws. Shadow environment secretary Sue Hayman said: "Michael Gove and Theresa May cannot stick their heads in the sand forever. There are serious questions about the supply, affordability and quality of food we can expect after we leave the EU."
Conservative MP and chair of the Environment, Food and Rural Affairs Committee, Neil Parish, said that UK farms were facing an "agricultural crisis".
"It is unacceptable that perfectly good food is rotting in fields. At a time of uncertainty around our future trading relationships, we should be increasing our domestic food security rather than our reliance on imported foodstuffs," he said.
"The government is not acknowledging the scale of the issue."