This latest success is an intravenous infusion of two monoclonal antibodies known as REGEN-COV.
The results from the trial suggest that for every 100 patients treated with the antibody combination, there would be six fewer deaths.
"We are very proud to have contributed to this fantastic effort," said Dr Martin Knolle, Addenbrooke's principle investigator, Recovery trial.
“This important finding adds further treatment options to those already shown to be effective by the Recovery trial against a disease no one had heard of a year and a half ago."
The combination of the REGEN-COV antibodies bind on to the virus, stopping it infecting cells and replicating.
Only those who have not already made any antibodies of their own to fight the virus should be given the treatment.
In the trial, the treatment was given in addition to the anti-inflammatory steroid drug dexamethasone, the first effective treatment discovered by the Recovery trial, which itself cuts death risk by up to a third for the sickest Covid patients.
"It's the first time any antiviral treatment has been shown to save lives in hospitalised Covid-19 patients. "
Sir Martin Landray, joint chief investigator, said: "We now know that this antibody combination is not only bad for the virus but it is also good for the sickest patients who have failed to mount a natural immune response of their own.
"We are incredibly grateful to the many NHS staff and patients who have contributed to today’s discovery."
Fellow joint chief investigator, Sir Peter Horby, said there had been great uncertainty about whether antibody therapies were the right approach, when some other studies had found no benefit.
But the antibody treatment used in the Recovery trial contains large doses of two specific antibodies, made in the lab, that are good at latching on to the pandemic virus.
Sir Peter said: "These results are very exciting. It is wonderful to learn that even in advanced Covid-19 disease, targeting the virus can reduce mortality in patients who have failed to mount an antibody response of their own."
In February, the Recovery trial also found that the anti-inflammatory treatment, tocilizumab, reduces the risk of death when given to hospitalised patients with severe Covid-19.
Tocilizumab is a drug normally used to treat rheumatoid arthritis.