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The National Trust has weighed into the campaign to keep subsidies for Britain's farmers at the current level.

It says the countryside faces a decade of damaging uncertainty in the wake of Brexit unless the government establishes support policies soon.

The Trust said affordable, high-quality food and wildlife-friendly farming can be secured for the current subsidy of £3bn a year.

Other commentators query whether that much cash needs to be spent.

They wonder whether the environmental objectives can be achieved more cheaply.

Currently only a fifth of farm subsidies in the UK goes to support the environment and rural development, so even transferring £1.5bn to the environment would provide a major boost.

But the Trust’s director, Helen Ghosh, said farmers needed reassurance that food standards and environmental protections will be maintained with a promise of the full £3bn to be spent.

“Current uncertainty is prompting some farmers to revert to intensive methods for short-term profits, damaging long-term agriculture and dwindling wildlife”, she warned.

"We have already seen examples of short-term decision-making, where farmers, in response to uncertainty about the future and income, have ploughed up pasture which was created with support from EU environmental money.”

Lost gains

The question of how much cash is needed to protect wildlife and provide “ecosystem services” like flood prevention and water catchment is debated.

The consultancy Agra Europe has previously concluded: "What is certain is that no UK government would subsidise agriculture on the scale operated under the CAP."

Warwick Lightfoot from the right-leaning think tank Policy Exchange said the current system was so inefficient that environmental objectives might be able to be achieved for less than £3bn.

But a former senior farm specialist at Defra, Martin Nesbit, told BBC News that if the government was serious about wildlife targets, the full £3bn annually is probably needed.

Mrs Ghosh will tell the BBC Countryfile Live event at Blenheim Palace: "We are within touching distance of a vision for the future of farming that sees thriving businesses successfully meeting the needs of the nation into the 21st century and beyond.

"The longer we wait, the more we risk losing all the gains we have made over the last decade."

The Trust itself is a major beneficiary of grants for its wildlife-friendly farming.

The Country Land and Business Association (CLA) recently proposed a new system under which land-owners get paid for contracting to look after the countryside. The farmers’ union NFU is also poised for change.

There are huge questions, though, about how subsidies should be shared and administered; and about what sort of countryside people want.

Re-wilding campaigners want much of the uplands to be left unsubsidised - to return to forests that will catch flood waters and nurture wildlife without public subsidy.

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