A senior Conservative has called for a rethink of his own party's "calamitous" Brexit policy, warning a trade deal with Donald Trump's America would wipe out Scottish farmers.

Former MEP Struan Stevenson raised the spectre of falling subsidies and collapsing land prices while imports of cheap low-quality hormone-treated beef and chlorine-washed chicken destroyed domestic production.

Writing in The Herald, the veteran Tory echoed grave concerns expressed in this newspaper's Beyond Brexit series last month by both economists and farming analysts.

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Agenda: Struan Stevenson on how Rural Scotland is facing a post-Brexit meltdown

Mr Stevenson said: "When a hard Brexit means hurtling over the cliff’s edge and a soft Brexit means paying for EU membership but having no say, it is time to call a halt.

"Let us hope that Scotland’s farmers will add their voices to the growing nationwide clamour for a re-think on the calamitous course that the UK now seems to be pursuing."

His intervention comes amid open speculation that some of Theresa May's cabinet are looking for a way to reverse last year's Leave vote as the sheer economic, social and cultural damage it will wreak becomes increasingly clear.

Mr Stevenson, however, is one of very few Conservatives to break cover and make an outright call for the UK to remain in the European Union and the single market Britain did so much to create.

Agenda: Struan Stevenson on how Rural Scotland is facing a post-Brexit meltdown

The MEP for 15 years was voicing concerns over the fate of Scottish farming that have heightened ever since his party colleagues talked up the prospect of post-Brexit trade deals with English-speaking former colonies, such as American, Canada and New Zealand.

At the summit of the G20 group of industrialised nations US President Donald Trump this weekend spoke of a new agreement on trade with Britain "very quickly". Previously industry group Quality Meat Scotland warned that such a deal would mean a "downward spiral" for Scottish farmers who have specialised in high-quality healthy grass-fed beef which cannot compete on price with US meat. Such competition, Mr Stevenson added, would come just as Scottish farmers faced potential barriers to their trade with traditional export markets in the EU.

The former politician, now chief executive of pro-union group Scottish Business in UK, wrote: "The Brexiteers also claim that the EU’s protectionist policies discriminate against cheap food imports and force up food prices for British consumers.

Agenda: Struan Stevenson on how Rural Scotland is facing a post-Brexit meltdown

"In other words they want cheaper food following Brexit. That means throwing open UK markets to cheap food from Africa, Australia, North America, Brazil, and Argentina, causing chaos for UK farm gate prices, a further fall in land values and widespread bankruptcies. UK food self-sufficiency would plummet."

Farmers, he said, would face this price crunch as subsidies - currently paid by the EU - came under pressure from an austerity-driven UK Government. "Only the super-efficient, top 10 per cent of farm businesses could survive without them. Most farmers have thin margins, if they have any margins at all. The European Commission estimates that land prices would fall by 30 per cent if farm subsidies were totally abolished in the UK and they would fall sharply if subsidies were reduced. For farmers who have taken out bank loans against the value of their land, a loss of value could be fatal."

The National Farmers' Union echoed Mr Stevenson's concerns. Its Scottish chief executive, Scott Walker, said undoing four decades of EU agricultural policy would be a "significant challenge" and warned lower standards from new trading partners like America would mean poorer-quality food on the nation's shelves.

Mr Walker said: "We do not underestimate how difficult it will be to get a trade deal with Europe that continues to give us unfettered access to their market but it is vital that we achieve one. The EU is Scotland’s most important export market after the rest of the UK and it is vital, for farming and crofting, that tariff and barrier free access to Europe remains for our food and drink industry.

Agenda: Struan Stevenson on how Rural Scotland is facing a post-Brexit meltdown

“Unless farmers and crofters either receive much more for their products, or are supported to at least the current level, then the whole of the UK will become reliant on imported food. The result of this would be that we have no food security and no ability to feed our own country.

“Instead we would become totally reliant on others selling to us and the standards of food bought in supermarkets would be determined by the production standards in other countries rather than the high standards of production we set ourselves.”