The drug, called VIR-7831, is a new treatment for people with mild to moderate disease, and the study was so successful that it was stopped early.
GSK and its partner, Vir Biotechnology, plan to immediately seek emergency use authorization in the United States and approval in other countries, potentially including the United Kingdom.
Monoclonal antibodies are molecules produced in the laboratory that mimic human antibodies.
The phase 3 global clinical trial based its initial analysis on data from 583 patients at risk of hospital admission.
GSK said VIR-7831 works in two ways – by blocking the virus from entering healthy cells and also by eliminating infected cells.
A separate laboratory study found that VIR-7831 is effective against the main current variants of Covid-19, including the Kent, South African and Brazilian variants, the company said.
VIR-7831 is designed to be administered as a single intravenous (IV) infusion.
Dr. Hal Barron, scientific director at GSK, said: “We are pleased that this unique monoclonal antibody was able to bring such a profound benefit to patients.
“We are looking forward to the possibility of making VIR-7831 available to patients as quickly as possible and to further explore its potential in other environments.”