Ministers have been told it is the job of the government and not schools to ensure that food supplies remain in place for pupils after Brexit.
Tes revealed last week that Lord Agnew has written to councils, multi-academy trusts and private schools asking them to contact their suppliers to ensure they will still be able to provide food if the UK were to leave the European Union at the end of next month without a deal.
The letter has been strongly criticised by school leaders and charity campaigners.
Paul Whiteman, the general secretary of the NAHT headteachers' union, said: “It is not reasonable to expect school leaders to ensure food supplies to schools post-Brexit when the government is not able to do so for the country itself.
“There is enormous uncertainty facing schools and businesses over Brexit, and no one is currently able to give any guarantees.
"Instead of passing the problem over to individual schools to try and solve, the government must urgently ensure that vulnerable children are protected from any interruptions to food or medicine supplies, and provide the proper clarity and reassurances to schools and to families about how Brexit will affect them."
It was revealed earlier that a no-deal Brexit may cost schools up to £85 million a year in additional food costs alone, according to a leaked Department for Education analysis of the risks. The Local Government Association has warned that any increased cost in providing food services to schools as result of Brexit should be met by the government.
Councillor Kevin Bentley, the chair of its Brexit taskforce, said: "The LGA will continue to identify the issues which need to be addressed at a national level, including the need to ensure the continuity of supplies to meet the government’s school food standards.
"The government also needs to make sure any added responsibilities as a result of Brexit are fully met and sufficient funding is available."
Lord Agnew’s letter urges local authorities and academy trusts to respond to a survey by the end of this week to give the department a better understanding of school preparedness for Brexit.
Kath Dalmeny, the chief executive of the Sustain food charity, described the letter as “extraordinary” and accused the government of attempting to pass its responsibilities onto schools.
She said: “In August, government ministers said that food provision for schools was led by the Department for Education, and yet here they are passing the buck to local authorities, the food industry and, incredibly, individual schools all in one message.
“The food industry has already made it clear that, in the event of a no-deal Brexit, it is anticipating additional border checks that will lead to fresh food shortages and price rises – those checks are beyond their control, so how they are supposed to guarantee supplies to schools – or anyone else – at this stage is anyone’s guess.
“The government simply can not hide on this issue any longer. Just who is responsible for food supplies in the event of a no-deal Brexit?
"How do they intend to ensure schools, hospitals, care homes and frontline charities get the food they need to feed the vulnerable, and how do they intend to cushion the blow of food shortages and price hikes to those on low incomes?”
Geoff Barton, general secretary of the Association and School and College Leaders, has said: "Lord Agnew's letter puts an onus on local authorities and schools to prepare for Brexit while giving them only the vaguest idea of what to expect.
"There is little in the way of practical advice other than telling them to contact their food suppliers to check what arrangements have been put in place over the supply of school meals.
"It is difficult for schools and colleges to develop any meaningful contingency plan to prepare for Brexit when the Government has little idea about what will happen on 31 October."
A spokesperson for the government said: “We have a highly-resilient food supply chain, and this will not change when we leave the EU.
“The food industry is experienced in dealing with scenarios that can affect food supply, and we are working closely with suppliers to ensure provision to schools remains unaffected.
“We are confident that schools will continue providing pupils with nutritious school meals, as required by the School Food Standards.”