A pill to treat Covid developed by the US company Pfizer cuts the risk of hospitalisation or death by 89% in vulnerable adults, clinical trial results suggest.
The drug - Paxlovid - is intended for use soon after symptoms develop in people at high risk of severe disease.
It comes a day after the UK medicines regulator approved a similar treatment from Merck Sharp and Dohme (MSD).
Pfizer says it stopped trials early as the initial results were so positive.
The UK has already ordered 250,000 courses of the new Pfizer treatment, which has not yet been approved, along with another 480,000 courses of MSD's molnupiravir pill.
Health and Social Care Secretary Sajid Javid called the results "incredible", and said the UK's medicines regulator would now assess its safety and effectiveness.
"If approved, this could be another significant weapon in our armoury to fight the virus alongside our vaccines and other treatments," he said.
The Pfizer drug, 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.
Three pills are taken twice a day for five days.
The combination treatment, which is still experimental because trials haven't finished, works slightly differently to the Merck pill which introduces errors into the genetic code of the virus.
Pfizer said it plans to submit interim trial results for its pill to US medicines regulator the FDA as part of the emergency use application it started last month. Full trial data has not yet been published by either company.
The US has already secured millions of doses of the pill, according to President Joe Biden.
The company's chairman and chief executive Albert Bourla said the pill had "the potential to save patients' lives, reduce the severity of Covid-19 infections, and eliminate up to nine out of 10 hospitalisations".