The Tories have pledged to force the public sector to reveal how much food is sourced locally, said shadow DEFRA secretary Nick Herbert. The rule would affect schools and hospitals amongst other public bodies.

The pledge came as Mr Herbert launched the Tories' Honest Food campaign, which also calls for clear country of origin labelling, and a code to provide a clear, simple and workable set of standards for the public sector when purchasing food, for which more than £2bn a year is spent.

Local councils and NHS organisations would have to publish details of food bought, so people could hold them to account and demand improvement.

"Taxpayers had no idea how much of the food served in their local school or hospital was British," Mr Herbert said.

"We need a government that backs British farmers, takes a lead in sourcing more local food and is transparent about what food the public sector is buying." .

"More local sourcing is good for the consumer, good for British producers, good for the environment, and can be good for the bottom line as well."