What makes a good citizen?" Gordon Brown was not the first to anguish over this question. Back in 15th-century Italy, the attempt to answer it effectively established education as a force for change in the west. Yet, as the very word "renaissance" suggests, the project to explore and define what civic identity might be drew its truest inspiration from the distant past. "As to rebellion in particular against monarchy," grumbled Thomas Hobbes, in the wake of Charles I's execution, "one of the most frequent causes of it is the reading of the books of policy, and histories, of the ancient Greeks, and Romans."

That the old reactionary was not exaggerating would be demonstrated by both the American and French revolutions, and by the rise, in more recent times, of mass democracy, all of them initially inspired by classical models of citizenship. Indeed, it is no exaggeration to say that the study of ancient history has served, over the centuries, as the midwife of almost everything that makes the west politically distinctive today.

In modern schools, of course, history tends to mean Hitler. A simple lesson is served up to students on a plate: fascism is bad. The political and moral ambiguities of classical history, which inspired Machiavelli and Shakespeare, Jefferson and Marx, barely intrude upon the classroom. Students who wish to study Greece and Rome - no less the bedrocks of our own civilisation now than in previous centuries - already find it difficult enough. Next year, however, they will find it impossible. OCR, the single examination board in the country to set an A-level in ancient history, has decided to abolish it. Any student inspired by seeing 300 in the cinema, or watching Gladiator on DVD, or playing Rome: Total War on a PC, will soon have nowhere to go.

At a time when the profile of classical history has never been higher in the mainstream media, and when the uptake of the AS-level alone has tripled since 2000, it seems an act of near lunatic irresponsibility to prevent students from studying a discipline that actually enthuses them. Well might there have been howls of anguish from teachers, a debate in parliament, and even a Downing Street e-petition.

Admittedly, when one lists all the problems faced by the world, the fact that for the first time since the Renaissance British schools will no longer be teaching ancient history might not seem to rank very high. Nevertheless, with its peculiar blend of penny-pinching, philistinism and misplaced utilitarianism, OCR is taking a terrible wrong turn - and an unfortunately timed one too.

As the chancellor is certainly not alone in reminding us, issues of citizenship have recently become one of the hottest of political hot potatoes. Hence all the muddied debate about "British values", and the government's determination - once we have decided what these values might be - to incorporate the study of them into the national curriculum. And yet, as Baljeet Ghale, the president of the NUT, recently pointed out, there is no definition that could possibly satisfy the entire spectrum of current national opinion. As a result, any curriculum which includes them is bound to be grotesquely politicised.

We are not the first society, however, to have been faced with this problem. The reason why, in the past, ancient history was studied with such urgency and passion was precisely because it was recognised by so many educationists as providing the perfect solution. Most of the civic values that Alan Johnson has said he wants to see promoted in schools - from free speech to respect for the rule of law - derive ultimately from Greece and Rome; but the classical world is nevertheless sufficiently remote from us to be politically neutral.

Students who study it will rarely find themselves being given easy answers. Yes, Athenian democracy was a glorious and heroic achievement; but was it dependent for its vibrancy upon overseas adventures and exploitation of the disenfranchised? And yes, Roman citizenship was a stirring ideal; but did the liberties of the republic end up inevitably breeding autocracy? To ponder these questions is to find a whole line of political inquiry opening up before you - a line that leads directly to the present day.

Far from abolishing the ancient history A-level, then, OCR should be promoting it as hard as possible. As it is, they are squandering a golden opportunity. O tempora, O mores, as Cicero would doubtless have exclaimed.