Farmers have written to the bosses of the UK’s “big six” supermarkets, calling on them to end “farmwashing” and support local producers.

The Farmers against Farmwashing campaign, led by Riverford Organic vegetable box company founder Guy Singh-Watson, claims major supermarkets are using fake farm brands and overuse of Union Jack branding to give shoppers the impression that their products come from “quaint British family farms”.

It claims that in reality, much of the food is increasingly sourced from “industrial US-style, mega farms” or from overseas, and masking the decline of Britain’s family farms.

A survey of 202 farmers in July who are currently trading with supermarkets, or have done within the last five years, found that 61 per cent are concerned they will have to give up their farms within the next 18 months.

Just one in four (25 per cent) said they believed claims from supermarkets of supporting British farmers were credible and backed up by their buying practices.

The poll suggests that two thirds (67 per cent) of farmers feel under pressure from supermarkets’ buying practices and 68 per cent fear they will be de-listed if they complain about buying behaviour.

A separate survey of consumers found 74 per cent want transparency around the sourcing of their food, while 67 per cent felt “distrustful” or “annoyed” when they learned that the word “farm” can be used by supermarkets without it being from a real farm.

The poll found that 60 per cent of shoppers actively look for the Union Jack on products to support British farmers.

The open letter to supermarkets has been signed by more than 100 figures from the farming and food industry, including the MPs Cat Smith, David Chadwick and Ben Lake, the television presenter Jimmy Doherty and chef Rick Stein, as well as industry bodies such as Sustain, the Food, Farming and Countryside Commission (FFCC) and the Soil Association.

It is also accompanied by a documentary series which claims to “uncover the truth” behind claims of some British farm produce on supermarket shelves, which will be posted on the stopfarmwashing.co.uk site.

Farmer and TV presenter Jimmy Doherty said: “Britain’s small-scale farmers are facing extinction, pushed to the brink by the rise of US-style mega-farms. These enormous operations might make meat cheaper, but the hidden costs are devastating compromises on animal welfare, environmental harm, and the destruction of traditional farming livelihoods.

“We’re seeing generations of family farms swallowed up, replaced by faceless corporations that are driven by profit rather than people or the planet. It’s time we take a stand and support real British farming before it’s too late.”

Mr Singh-Watson said: “British farming is at a breaking point. The public cares deeply about where their food comes from – the supermarkets know this and they are using that trust to steal farmer stories and to hoodwink shoppers into thinking they are buying from those small-scale, traditional British farms.

“Yet the reality is that these farms are being pushed to the brink. Is this the future we want for our food system and our countryside?

“Farmers are already struggling with weather extremes, labour shortages, and rising costs, and while supermarkets claim to support British farming, they fail to back it up with their buying practices.

“Supermarkets must stop exploiting farmers and start supporting them. If we want a future where our food system is resilient, our landscapes are preserved, and our farmers are treated fairly, we need more honesty and decency in the supply chain.

“The British public has shown they care and would pay a little more if they knew it supported better farming. It’s time supermarkets and the Government listened.”

Andrew Opie, director of food and sustainability at the British Retail Consortium, said: “Supermarkets are always keen to promote British farming and follow the strict legislation on the labelling of food, including the use of flags. In fact, they pushed the Government to tighten these rules earlier this year.

“Retailers create new farm brands for some of their own-label goods in order to help consumers find the quality goods they are looking for.”

Censuswide surveyed 202 farmers between July 8-16, and 2,003 UK adults between July 8-10.