Professor John Watson, head of the HPA respiratory diseases department, said the third case followed the confirmed diagnosis of two cases announced this week. One family member has traveled outside of Britain.
A patient from Qatar was previously diagnosed in Britain with the virus.
Corona viruses are causes of the common cold but can also be associated with more severe illness, such as Severe Acute Respiratory Syndrome.
The patient in the new case, a British resident with no history of recent travel, is recovering from a mild respiratory illness and is currently well, Watson said.
"Although this patient had a mild form of respiratory illness, as a precaution the HPA is advising that the patient self-isolate and limit contact with non-household members. Follow up of other household members and contacts of this case is currently underway," Watson said in a statement. "Although this case appears to be due to person-to-person transmission, the risk of infection in contacts in most circumstances is still considered to be low. If novel coronavirus were more infectious, we would have expected to have seen a larger number of cases than we have seen since the first case was reported three months ago."
The case brings the total number of confirmed cases globally to 12 -- including five in Saudi Arabia, two in Jordan and one in Germany.