Food supplies for the year would run out today if the UK could only rely on domestic produce, farmers have warned.
Falling self-sufficiency means Britain produces less than two-thirds (62%) of the food the country consumes, down from 75% in 1991, the National Farmers' Union said.
If all the food produced in the UK in a year were stored and eaten from January 1, the "cupboard" would be bare by August 14, the NFU has calculated.
Farmers are calling for support from politicians, the public and food industry to back British farming and help them produce more.
NFU president Peter Kendall said the UK could not simply go around the world chasing the cheapest deal on food.
"To think that today's date would signal the time when our domestic food supply runs out is frankly alarming. It says to me that we must act," he said.
"Right across the board farmers have a fantastic natural capacity to produce more British food, given the right market signals and the confidence to invest. We have the right technologies to produce more from less, with precision farming helping to target fertiliser and crop protection products within centimetres.
"Laser technology can even pinpoint an individual weed, improving accuracy and efficiency. Crops grown under cover help to lengthen the season for our British fruits.
"But there is more to do to empower our farmers to enable them to make the most of our natural resources and feed our growing nation."
He urged the Government to help create an environment where farming businesses could invest, to address market failures and iron out price volatility to ensure the food chain can increase supplies.