A UK Parliament committee warns that farmers can face tariffs of up to 50 per cent on exports to the EU after Britain leaves.
A report, from the environmental audit committee, said leaving the EU posed risks for UK farming, the countryside and wildlife unless ministers took concerted action to maintain subsidies and standards.
It is also warning the British government that environmental protections must not be weakened.
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The UK government has guaranteed to maintain committed EU funding for farmers until the end of the decade - but not beyond.
Speaking on Tuesday, committee Chair Mary Creagh said, "Changes from Brexit could put our countryside, farming and wildlife at risk.
"Protections for Britain's wildlife and special places currently guarded under European law could end up as 'zombie legislation', even with the Great Repeal Bill".
EU subsidies currently make up more than half of farm income and it is not yet clear whether similar grants will continue after Brexit.
The report suggests that in future they should be linked to the promotion of biodiversity, preventing flooding and storing carbon.
It also claims that outside the single market sheep exports could face tariffs in excess of 30 per cent and 50 per cent for beef.
In order to avoid that happening, the UK National Farmers' Union (NFU) insists Britain's trade deal must be right.
Though it acknowledges that Brexit could be an "enormous opportunity," the organisation is campaigning to stay within the single market, especially as the majority of farmed exports are sent to other parts of Europe.
According to the Westminster committee, some of Britain's most pristine countryside is at risk from government efforts to boost the economy in the wake of Brexit.
Infrastructure projects and large developments are planned in a number of rural areas including the Lake District, the Cotswolds and Sussex's High Weald.
Projects in the pipeline include a new motorway between Oxford and Cambridge, new railway infrastructure in the Midlands and thousands of new homes.
A UK government spokesperson told this web site, "The government insists it is determined to get a good deal for the UK, not least for the food and farming industry, which it believes is a key part of the nation's economic success."