Education legislation passed last year allows parents to petition for ballots on whether schools should remain as grammars or whether selection should be abolished.

But the government has ruled out extra funding for school reorganisation where parents vote to end the selective system. As such, the Conservatives argue, money will have to be taken from school budgets to fund the change of status.

The figure of £500m is drawn from a projection based on Kent County Council's estimate on changing its secondary system from selective to comprehensive. Kent has 33 of the over 160 grammar schools in England and says reorganisation would cost £150m.

The Shadow Education Minister, Damian Green, said: "This is a scandalous waste of valuable funding. The government, it appears, are willing to throw away hundreds of millions of pounds in order to destroy the very schools which deliver some of the highest standards in the country.

"If the government sticks to this line, then the money to cover this transitionary period will have to come from front-line services - books, teachers and classroom equipment.

"This will damage the education of every child in the affected areas, not just those at grammar schools."

A spokesman for the Department for Education said the projection of £500m was "excessive. We do not recognise these figures".

There are no grammar schools in Scotland and Wales, but Northern Ireland retains selective schools. More than half of the remaining grammar schools in England are in the south east and London.