Elements of ancient history will be in a new classical civilisation A-level.
Shadow education minister Boris Johnson said: "You might as well say you can learn English history through the study of English language and literature."
Mr Johnson - who presented a BBC TV series on the Roman Empire - said: "The birth of Athenian democracy, the transition of Rome from republic to empire: these were critical events in the shaping of our civilisation.
"How can we understand ourselves if we cut ourselves off from our past?
"You can't just subsume the study of ancient history into the study of classical civilisation."
Mr Johnson, president of the Joint Association of Classical Teachers, added: "If we lose ancient history A-level, we lose yet another battle in the general dumbing-down of Britain." It was, he said, "a tough, rewarding, crunchy subject" giving way to "the softer option".
It was also perverse, given the immense interest in the Persian Wars, and the success of the film 300.
A spokesperson for the board said it remained committed to classics and was the only board still offering "a comprehensive suite of A-levels”.
