Jeremy Corbyn's plans to impose VAT on private schools to fund free meals for primary schools has come under fire after questions over the evidence were raised.
The Labour leader is due to explain how a universal rollout of free meals will help to increase pupil productivity and health, ending what he calls a "privileged subsidy" at the same time.
Shadow education secretary Angela Raynor told Radio 4's Today programme about the policy, is based on a 2012 report from the National Centre for Social Research, as well as from the Institute for Fiscal Studies.
However, one of the co-authors of the report - based on research conducted amongst primary school students in Newham and Durham - has disputed this claim.
In an interview with the same programme, they said while there had been a "minor improvement", they weren't sure if the effect would be the same if the policy was rolled out across the UK, and said such a roll-out would cost £1 billion.
The policy is expected to be announced in Lancashire later on Thursday.