Once again, the issue of school vouchers is back on the political agenda.

The Conservatives have been calling them “pupil passports” and focusing on how they could widen choice in state-funded education.

Yet if some privately-run schools were given state-level funding for each pupil, a crucial issue would be whether parents could “top up” vouchers with fees. Last week shadow chancellor Oliver Letwin wondered out loud whether this might be permitted, for relatively low-fee schools serving modest-income families, but not to subsidise children attending Eton.

This may at first glance seem a fairer way of supporting private education than fully-fledged vouchers subsidising all families who pay for private schools, including the extremely well-off 7 per cent who do so at present.

Letwin’s idea appears less socially unjust in terms of transferring money to the rich. In fact, vouchers allowing limited top-ups are particularly dangerous in their potential social and educational consequences. Imagine a future in which the state gives pound;3,500 per pupil (the cost of state education) to any school - public or private - provided it does not charge fees above a certain limit, say pound;2,000. And suppose that under such a system the main consideration for parents was class sizes, which were roughly proportional to per-pupil spending.

In this case, different tiers of schools would serve different markets.

Some might charge pound;1,000 to modestly well-off families keen for their children to have a little more teacher attention than in a “bog-standard” school.

Other schools might aim themselves at a more affluent clientele able to afford pound;2,000, which would perhaps buy class sizes of 19 rather than 30. Both markets would be much bigger than the small group which at present buys private education without state help.

Crucially, the no-fee sector could gradually deteriorate into a refuge for the poor to serve only those families with no spare resources to top up the quality of their children’s education. Is this scenario far-fetched? Experience in Australia shows that it is not. In the 1970s, a tiered voucher system was set up to rescue private Catholic schools, most of which served relatively poor communities and charged minimal fees. Each private school was given something, but less if it commanded high-fee income.

The result has been a steady growth in independent schools with modest fees which appeal to middle-class families who want to escape the public sector but cannot afford the full cost of doing so.

Independent schools have been boosted lately by more favourable subsidies introduced by a conservative administration. Today, a third of Australian children attend private schools, up to 40 per cent in the secondary sector in some states.

The slow but relentless growth in the market share of independent schools is steadily demoralising the public sector, not helped by recent experience of more generous funding for private schools while public resources stagnate.

For all the travails and crises facing state schools in this country over the past 25 years, the percentage of students going to private schools has remained stable at 7 per cent. That has helped to keep the improvement of state education a nationally shared priority.

Of course, the Conservatives will claim to retain that priority, and any help for private schools will be presented as a socially progressive measure focused on improving options for relatively worse-off families.

Yet however it were designed today, this could be a first step towards helping those who can afford it to buy their way out of state education rather than remaining and improving it.

Small steps towards segregating students can so easily be followed by larger ones. When the Tories introduced specialist schools in a 1992 White Paper, they allowed them to select 10 per cent of pupils on aptitude, and said this was not about reintroducing wider selection.

Four years later, the Conservatives produced another White Paper allowing specialist schools to select 30 per cent, grant maintained ones 50 per cent, and any school 20 per cent of their pupils. Had these proposals been put into practice, they would have marked the end of comprehensive education.

One reason we have escaped vouchers for private schools in this country is that they could be extremely expensive. Today, as in the Thatcher era, rebating the cost of a public education to existing fee-paying parents would be tough for a party committed to cutting taxes.

Now, a clever shadow chancellor is considering a version which would avoid that expense - by denying vouchers to today’s expensive private schools and aiming them principally at those already being educated by the state.

This financial feasibility makes the idea of top-up vouchers a particularly alarming threat to state education.

Donald Hirsch is the author of a wide range of international studies of school choice for the Organisation for Economic Co-operation and Development Another voice 22