Food production in the UK will be put at risk without government commitments to safeguard farmers after Brexit, and food scares could be the result, the head of the UK’s biggest farming organisation has warned as ministers announced the biggest shake-up to farming in decades.

The agriculture bill was hailed by Michael Gove, environment minister, as a “historic moment” heralding a “brighter future”, but swiftly came under attack from farming leaders for prioritising environmental issues over food production.

They are concerned about the loss of the roughly £3bn a year farmers currently receive from taxpayers under the EU’s common agricultural policy, and the threat of low-cost imports.

Minette Batters, president of the National Farmers’ Union (NFU), said: “To put an agriculture bill out there before we know the terms of trade with other nations is extraordinary.”

“You have to ask the question: who will be producing our food?” she said. “Looking to the rest of the world to shore up what we do seems a massive error.”

Batters raised the spectre of the food scares of the 1990s in warning of the threats of imports from countries with lower quality standards. “Have ministers forgotten the lessons of BSE and salmonella?” she asked.

Under the agriculture bill, outlined by Gove on Wednesday, farmers would be rewarded for signing up to “environmental land management contracts”, which would commit them to measures protecting wildlife, soil, water and air quality. The new system would be phased in from 2021 to 2027, with the government providing subsidies along the same lines as the EU CAP up to 2021.

Crucially, however, the government refused to give any assurances on how much public money would be available for environmental schemes, and whether there would be any taxpayer-funded support for food production.

Batters said there was a “clear market failure in the food chain”, meaning that most farmers were not receiving enough for their produce to cover their costs without public subsidy. She said British consumers had benefitted from taxpayer support for farming, and that food bills in the UK were cheaper than average. “Investment of £3bn [a year] in food and the environment is a phenomenally good deal for the British consumer,” she told a press conference on Wednesday. “Safe affordable food is good for the public.”

Highlighting the problems faced by farmers over the heatwave and drought this summer, she warned that, in announcing the bill the government had “made no mention of volatility” and how to support farmers through such unpredictable events.

While the NFU supported moves to encourage farmers to protect the environment, Batters argued that ensuring farmers can produce food profitably was the essential first step. “You cannot go green if you are in the red,” she said. “The number of farmers in the red is at a record high.”

Some sectors of farming are particularly at risk. Sheep farmers, for instance, often struggle with poor land and low prices for their produce. Phil Stocker, chief executive of the National Sheep Association said: “Farmers would prefer to be in a position where they are not reliant on support payments, but market mechanisms, and many farms’ structures just do not allow for that at the moment.

“Maintaining the provision of nutritious, quality food is in the public interest and we don’t believe this can be achieved by leaving it to the vagaries of a purely profit-driven marketplace.”

The NFU’s strongly worded attack on the agriculture bill is an unusual step, as for many years successive governments were careful to court farming leaders. The breach highlights the uncertainties that Brexit is bringing to agriculture, with farmers worried that new tariffs could destroy their export market, and cheap imports would flood supermarket shelves, all while their EU subsidies disappear and seasonal workers become harder to hire.

Batters has previously warned that a no-deal Brexit would spell “armageddon” for the UK’s food supplies.

Green campaigners welcomed the government’s focus on public goods, but some called for a broader definition that would include animal and human health as well as environmental issues such as water and air quality. Vicki Hird, food and farming policy coordinator at Sustain, said: “We don’t just want high quality and traceable food, but a farm policy that clearly supports public health goals, for example by encouraging the production of more and diverse fruit and vegetables, and high welfare livestock systems that don’t rely on antibiotics.”

Without details of the funds to be made available, it was difficult to judge the potential effects of the bill, said Graeme Willis, rural policy campaigner at the Campaign for the Protection of Rural England. “It remains to be seen whether there will be sufficient money to support a diverse farming sector and fund the restoration of a healthy countryside and landscapes. Commitments to support new entrants are welcome, but there must also be measures to reverse the decline in smaller farms.”

Farmers who had already begun to include environmental protections in their work would be in a good position to take advantage of the reforms, said Martin Lines, chairman of the Nature Friendly Farming Network. “It will be challenging for many farmers to adapt, but many farmers have been leading the way for years and want to produce great food from a countryside bursting with wildlife. This bill is a crucial first step towards a better future for farmers and nature.”

Tanya Steele, chief executive of WWF, added that there was strong demand for a greater emphasis on environmental protection from farmers. The public overwhelmingly want farmers to be paid to protect our nature – over 90% according to a recent poll [we] conducted. This bill, if properly implemented, is a once-in-a-lifetime opportunity to make sure we have a farming policy that restores rather than destroys our natural land.”

Separately, Gove was also accused by campaigners and opposition party MPs of backtracking on his commitment to end the export of live animals. In an interview with the Today programme, he avoided explicitly confirming that a ban would be introduced and spoke instead of “restrictions” on such exports. Compassion in World Farming, the NGO, said a full ban was needed to end animal cruelty, while Labour and the Green party condemned what they saw as wavering on the issue.