Remdesivir, which was used to treat Ebola, is produced almost exclusively by US pharmaceutical giant Gilead. It costs around £430 for a treatment course of six doses.
The US Department of Health and Human Services (HSS) has announced it has bought up more than 500,000 doses of the drug.
The HSS suggested that the move means the country has secured the entire global supply of the drug for July, and 90 per cent of stocks available for August and September.
The US’s decision to stockpile the drug means there will likely be little supply in the rest of the world for months.
Trials of the antiviral medicine on Covid-19 patients have showed it reduces the length of time they experience symptoms from 15 to 11 days, by stopping the virus from reproducing.
Speaking in May, Health Secretary Matt Hancock described a trial of the Remdesivir drug as “the biggest step forward” since the beginning of the pandemic.
Dr Andrew Hill, a senior visiting research fellow at Liverpool University, said it is now expected that Remdesivir will not be available for use on patients in the UK and Europe until October.
"This deal that's been struck by America means that people with COVID-19 in the UK can't get access to these treatments that would get them out of hospital quickly and might improve their chances of survival,” he told Sky News.
"So far, we know that for the next three months there will be no supplies of Remdesivir - America will take the drugs and we won't have access to them. That's the case in the UK and Europe."
Low and middle-income countries can produce generic versions of the drug, but they are not able to sell them to Europe because Gilead has a patent for it, he added.