The UK’s largest union has accused the government of suppressing information on possible changes in food prices as a result of Brexit.

Unite said it made a Freedom of Information (FOI) Act request to the Department for Environment Food and Rural Affairs (DEFRA), asking the department what assessment or estimate has been made of the increase in food prices in the run up to the UK leaving the European Union and the first five years after the UK’s departure.

But DEFRA refused to issue the information as it said it was exempt from the FOI request under section 35 of the act, which relates to the formulation and development in government policy.

“In applying this exemption, we have had to balance the public interest in withholding the information against the public interest in disclosure,” said DEFRA.

“We recognise that there is a public interest in disclosure of information concerning the increase in food prices in the run up to the UK leaving the European Union and the first five years after the UK’s departure.”

However, there was a “strong public interest” in withholding the information, as the UK was formulating its negotiating position with the EU, and needed “a safe space” to formulate policy effectively and to ensure the information it is preparing is timely and accurate, it said.

“DEFRA’s EU Exit policy development work is ongoing. We consider that premature disclosure of information could seriously mislead the public.”

Unite national officer for food, drink and agriculture, Julia Long said: “If the government knows that Brexit is going to affect food prices, then they need to tell the general public and not pretend that there isn’t a problem. “The type of Brexit that the UK chooses will clearly have major implications on the nation’s shopping basket and we need to know what those factors will be.”

National Farmers’ Union (NFU) president Meurig Raymond said: “We know the rise in inflation and food inflation will present its own challenges for people at home doing their daily shop.

“But the link between retail prices and farm gate prices tends to be weak and time-lagged and for any commodity, a combination of factors will influence pricing levels such as transport, energy, labour and packaging.”