Sexual health doctors said gonorrhoea was "rapidly" developing resistance.
A highly drug-resistant strain of gonorrhoea was detected in the north of England in March.
That strain is able to shrug off the antibiotic azithromycin, which is normally used alongside another drug, ceftriaxone.
In her letter, the chief medical officer said: "Gonorrhoea is at risk of becoming an untreatable disease due to the continuing emergence of antimicrobial resistance."