Shadow environment secretary Peter Ainsworth called for a "presumption" in favour of UK farm-assurance standards or their equivalent in UK food contracts. "This would make it easier for local suppliers to win business," said Ainsworth, who insisted departments "shunned" home-grown produce and missed a huge opportunity.
He unveiled figures last week that showed 75 per cent of apples, pears, lamb and fish served by the NHS were sourced from abroad.
Just 30 per cent of publicly procured apples, pears and plums were home-grown, down from 46 per cent from last year.
The proportion of British vegetables such as peas, beans, tomatoes, cucumbers, lettuce, leeks and celery bought across Whitehall had fallen sharply over the past year.
And the Department for Children, Schools & Families imported 98 per cent of its apples into the UK.
Ainsworth said: "In such difficult economic times the Government should be doing whatever it can to help British farmers, support small businesses and protect jobs.
"Spending a greater proportion of the public-sector food budget on good-value local produce has the potential to deliver benefits.
"Less than half of all Government departments and agencies currently demand farm-assurance standards, under which the vast majority of British food is produced. This opens the door to vast amounts of public money being spent on food that would be illegal to produce in this country.
"It is high time the Government moved towards a system where all publicly procured food meets British standards of production."
He highlighted examples of good practice. Royal Cornwall Hospitals Trust sourced 83 per cent of its food from local suppliers and cut annual food miles by 67 per cent.
Defra said it was "not permissible" for the public sector to actively promote British food under EU public-procurement law.
"Rules aim to open up the EU's public-procurement market to competition, prevent 'buy-national' policies and to promote the free movement of goods and services."
The Government's public-procurement policy also required purchases to achieve value for money and have "due regard to propriety and regularity", said Defra.