Farming leaders and landowners from across the UK have written to MPs to plead with them to make sure that the idea of a no-deal Brexit is taken off the table, warning of the “catastrophic” impact it would have on the country’s food supply.

They warned of a triple threat with the possibility of disrupted food supplies, higher food prices and farmers being put out of business because the EU market could be closed to British food exporters for six months.

“Brexit will mean that, for the first time in a generation, UK politicians will have direct responsibility for ensuring our nation is properly fed. The implications, not only for domestic food supply but for the careful management of our cherished countryside, would represent an historic political failure,” said the four main farmers’ unions, including the National Farmers Union, in a letter to MPs.

Separately, tenant farmers and landowners have written to MPs warning of a no-deal disaster.

“This is a recipe for disaster for all farmers and ultimately will cause long-term damage to the rural communities and countryside of our nation,” said the Country Land and Business Association and the Tenant Farmers Association in a joint letter.

They also want assurances over the prospect of lower-quality food such as chlorinated chicken, currently banned by the EU, entering the market in a no-deal scenario. They said the thought that the standards of British farming could be “undermined by cheaper, lower-quality, imports” was a major concern.

The NFU, NFU Cymru, NFU Scotland and the Ulster Farmers’ Union, said the impacts of no deal would mean a potential trade embargo on UK meat and plant products.

They raised similar concerns in September but feel that few grasp the implications of Britain becoming a “third country” in relation to the EU.

They have been told that 6,000 meat processing plants that export to the EU will have to undergo individual audits by British authorities which must then be certified in Brussels. These will then be checked by EU officials and put to a standing veterinary committee for approval, a process that the NFU has calculated will take six months “at a conservative reading”.

In its letter the NFU says this would lead to an “effective trade embargo on the export of UK animals and animal-based products”. Farmers exporting products would face “draconian tariffs” designed to make any non-EU products uncompetitive” against EU food. The effective EU tariff would be 65% on beef, 46% on lamb and 27% on chicken, MPs have been told.

Small-scale sheep farmers, many of whom earn less than £20,000 a year, are thought to be particularly vulnerable, with 21% of all lamb meat being exported and 94% of that to the EU. Almost 90% of all beef exports also went to the EU, according to 2017 figures.

The Farmers’ Union of Wales has suggested that mountain sheep farmers would be “wiped out” by a no deal over Brexit.