British farmers have been urged to "dramatically" increase productivity levels if they hope to survive after Brexit as they are failing to "keep pace" with European rivals and the United States.

After decades of relying on the EU subsidies, farmers must now boost efficiency to close a "yawning" productivity gap of £4.3bn in lost GDP, according to the Agriculture and Horticulture Development Board (AHDB).

"Currently, Britain is falling significantly behind major competitor countries in the rate of growth in productivity, with pace-setters like the USA and the Netherlands growing productivity three times faster than we are domestically," an AHDB spokesman told the Telegraph ahead of this week's Oxford Farming Conference.

He added: "A yawning productivity gap – worth over £4.3bn in lost GDP between 2000 and 2013 alone – has opened up as a result of the country failing to keep pace.