Speaking at a conference on admissions reform, Mary Curnock Cook, chief executive of the Universities and Colleges Admissions Service, voiced doubts over the accuracy of information collected by Ucas about students' social backgrounds.

The data - taken from university application forms - are used by institutions to decide whether applicants fall into the "widening participation" category and are eligible for lower-grade offers.

"We cannot access high enough quality information to make sure contextual data [are] serving [their] purpose," Ms Curnock Cook told the event in London on 22 May. "I have doubts whether contextual data [are] useful, even if the information is accurate."

For instance, data provided about students' eligibility for free school meals do not specify how long they had received them.

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"They might have had them for years or just a few weeks," she said.

Students might also be embarrassed to reveal details about their family's lack of wealth or educational achievement, she added.

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"This [may be] the first time you have been asked to say you live in a poor area, go to a rubbish school, [that] your parents are poorly educated and you have received free school dinners," she said.

Her comments were made on the same day that Nick Clegg, the deputy prime minister, spoke out in favour of contextual data. Alan Milburn, the former Labour health secretary, is also expected to back such indicators in his report on social mobility and universities, which is due to be published next month.

Ms Curnock Cook also questioned the government's decision to allow universities to recruit unlimited numbers of students who achieved AAB or better at A level.

"I believe it breaches institutional autonomy over admissions, and I am quite sure there will be some decisions made this summer based on whether the applicant falls into the AAB pot or non-AAB pot. That is a shame," she said.

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jack.grove@tsleducation.com.