Almost one million children from poor backgrounds will lose the right to free school meals if Theresa May pushes through cuts in the Conservative manifesto, an educational think tank has warned.

The Prime Minister announced last week that universal free lunches for infants will be stopped if the Tories win the June 8 general election, with free breakfasts on offer instead.

The move will cost families around £440 a year for each child affected and is thought likely to save around £650 million a year, according to the research by the Education Policy Institute (EPI).

The EPI found that those losing hot lunches would include 100,000 from families living in relative poverty, and 667,000 from those it defined as coming from “ordinary working families” of the kind that Theresa May has said she wants to help.

Those from the poorest backgrounds will still be entitled to a free midday meal.

EPI executive director Natalie Perera told The Observer: “Around 900,000 children from low-income families will lose their eligibility for free school meals under these proposals. Around two-thirds of those children are from what the Government considers to be ‘ordinary working families’.

“The typical annual cost for an ordinary working family would increase under these proposals to around £440 for each child aged between four and seven.”

Universal free lunches for infants were introduced under the coalition government by Liberal Democrat education minister David Laws, now the EPI’s executive chairman.

The party’s former deputy prime minister Nick Clegg said: “This just confirms the sleight of hand from the Conservatives – scrapping universal infant school lunches hits some of the most hard-pressed families the hardest. The offer of free breakfasts won't reach the children who don’t come to breakfast clubs.

“All Theresa May’s talk of helping the ‘just about managing’ will ring hollow as long as this regressive decision remains in place.”

But a Conservative spokesman said: “We don’t think it is right to spend precious resources on subsidising school meals for better-off parents. So instead we will give that money to headteachers, to spend on pupils’ education instead.

“We will make sure all those who need it most still get free lunches – and will offer a free school breakfast to every child in every year of primary school. So the most disadvantaged children will now get two free school meals a day rather than one.”

When the pledge was announced, Sarah Olney, the Lib Dem education spokeswoman, said: “Margaret Thatcher was know as the ‘milk snatcher’. Theresa May will go down as the ‘lunch snatcher’.”

Additional reporting by PA