In clinical trials the new drug reduced the risk of hospitalisation or death by 89% in vulnerable adults.
Paxlovid, known as a protease inhibitor, is designed to block an enzyme the virus needs in order to multiply. When taken alongside a low dose of another antiviral pill called ritonavir, it stays in the body for longer.
Interim data from clinical trials in 1,219 vulnerable patients who caught the virus found that 0.8% of those given Paxlovid were later hospitalised, compared with 7% of patients who were given a placebo or dummy pill. Dr June Raine, the chief executive of the UK Medicines and Healthcare products Regulatory Agency , said: "We now have a further antiviral medicine for the treatment of Covid that can be taken by mouth rather than administered intravenously.
The regulator said Paxlovid is most effective when taken in the early stages of infection and has recommended the drug be used within five days of the onset of symptoms.