Almost seven out of 10 new academies are falling foul of admissions rules by failing to make clear how they allocate their places, a poll by The TES suggests.
A survey of the websites of 200 new "converter" academies has revealed that 68 per cent have failed to post their draft arrangements for 2012.
Schools that act as their own admissions authorities - including academies, voluntary-aided (VA) and foundation schools - were legally required to publish their 2012 admissions arrangements online by 1 May.
But the figures for schools switching to academy status before May show they struggled to meet the deadline. Last year, just 53 per cent of secondaries and 28 per cent of primaries failed to hit the admissions deadline, according to figures from the Office of the Schools Adjudicator (OSA).
Failure to publish admissions criteria can leave schools open to appeals from parents who miss out on a place for their child.
A spokeswoman for the OSA said: "As the chief adjudicator has previously noted, many admission authorities, not just academies, leave their admissions arrangements open to challenge by failing to publish them by the required date."
At an admissions conference in February, chief schools adjudicator Ian Craig told heads: "Many of you don't (meet the 1 May deadline). If I was a person out there wanting to challenge your admissions, the first thing I would do is use that. And you'd lose."
Malcolm Trobe, policy director at the Association of School and College Leaders, said it was "absolutely clear" that schools have had their admissions sorted by 1 May.
"It could be that some are in a time lock on this, in terms of going through the procedures of their governing bodies, and waiting for the meetings of the appropriate governing committees," he said. "Sometimes the dates work out fine, sometimes they don't."
Mr Trobe said the delays might have been caused by schools waiting for the results of consultations on their admissions arrangements. "But it is a requirement for schools to make the information available electronically, and we would be expecting them to be working towards this," he added.
The Parkside Federation in Cambridge, which converted to academy status in April, has already posted its admissions arrangements for 2012 on its website, but executive principal Andrew Hutchinson said he had sympathy with schools that have not kept up to speed.
"It's a transitional period. The guidance academies used to get from local authorities may not be coming across in the same way it had in the past, and they might not have had to deal with this before."