The government wants exam boards to ensure that ancient history continues as an A-level subject.
The last board offering it, OCR, is proposing to stop doing so.
During a Lords debate, Schools Minister Lord Adonis said the government was "not content", and was inviting the boards to produce fresh proposals.
OCR's move has brought an outcry from classicists, including shadow education minister Boris Johnson, who wore a toga to a protest meeting this week.
An "e-petition" on the Downing Street website has attracted several thousand signatures.
During an earlier Commons debate, School Standards Minister Jim Knight said he was certain OCR and the qualifications regulator, the QCA, would resolve the issue "in a way that meets the needs of future students by ensuring that the qualifications offered cover a comprehensive and rigorous curriculum".
The government's stance appears to have hardened since then.
In the latest Lords debate, Lord Adonis said: "All periods of history, from the ancient to the modern, can inspire our young people to study the subject, opening them up to skills that are essential in today's world as well a vital understanding of our past.
"The government is not content to see the end of ancient history as a single A-level and has invited the exam boards to come forward with proposals for it to continue."
A Qualifications and Curriculum Authority (QCA) spokeswoman said it was discussing the situation with OCR and was arranging a meeting with ministers.
"There is no statutory duty that requires particular subjects to be offered, but regulators and awarding bodies work together to try to ensure that, where possible, low-entry subjects can be offered in a sustainable way, maintaining a wide range of options for learners."
- The International Baccalaureate Organisation (IBO) has apologised for setting a two-year-old paper in a history exam last week.
It said a "grave mistake" had been made in the reissuing of the paper, attributed to "human error".
A Welsh Assembly Government spokesperson said: "Senior managers at IBO fully realise the seriousness of this matter.
"Meetings will continue to be held with the IBO to ensure that candidates are treated appropriately and fairly and receive the grades they deserve."
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