It surfaced in a letter from an official at the Department of Education about a school in the London borough of Bromley. The official said that if the school wanted to select pupils on the basis of their ability in conventional academic subjects, that would amount to a significant change in the school's character and would need the specific approval of the Secretary of State.
He added, however: 'On the other hand, we take the view that the introduction of a degree of selection on the basis of ability in aesthetic subjects or sport need not be the subject of statutory proposals, provided that the proportion of places at schools allocated on this basis is no more than about 10 per cent.'
Critics argue that that will give middle-class children an advantage in seeking entry to popular schools and enable a school significantly to alter its intake. Mr Hart said yesterday: 'It will begin to pull the rug from under the comprehensive system. It is not just the individual school, but the impact that it will have on other schools or local authorities.
'It would reinforce all the fears of many of us, that the Government's policies will lead not to an extension of parental choice but a selection of pupils by schools.'
John Patten, Secretary of State for Education, wants schools to specialise in particular areas of the curriculum, so that they develop a reputation for excellence in, say, music, or in technology.
Martin Rogers, co-ordinator of Local Schools Information, the opting-out advisory body, said: 'As more schools opt out, it is increasingly difficult to monitor admissions policies and practices to ensure that children are treated fairly. Any relaxation of the rules can only make this worse.'
A department spokesman said that local education authorities have always had the flexibility to allow a small proportion of pupils to be selected for their talent in music, art or sport. But the Bromley letter suggests that Mr Patten will now encourage schools to select a share of their pupils as part of his specialisation policy.