The government began using the free school meal measure in August, citing problems with the quality of socio-economic data that had been used previously. It said that the free school meals measure was both "well established" and "versatile".
But Janet Graham, director of the Supporting Professionalism in Admissions programme, said that while university admissions offices can get data about the number of pupils receiving free school meals in a school, they do not have access to specific information about the individual. This meant they were working on the basis of "proxies".
While universities do often have data on family income to allocate bursaries, this can be shared with admissions offices only with an applicant's consent, she said. The situation was different in the US, where "universities know how many cars (applicants' families) have", she added.
Similar issues have been raised by Universities UK, which said there were problems with the free school meals measure, the most significant being that it "captures only a small proportion of pupils (14 per cent)".
Because universities do not know if their students come from this group, "they are not in a position to set milestones for recruitment of these students, nor evaluate the effects of any widening participation activity on entrants from this group", UUK added.