America has bought up almost the entire global supply of one of two re-purposed drugs used to treat coronavirus - depriving the rest of the world for around three months.
Remdesivir was first developed to treat ebola, but studies have found it is effective in improving outcomes for patients with severe forms of Covid-19.
The drug is produced by US pharmaceutical giant Gilead Sciences and was priced at $2,340 (£1,892) per patient in wealthier nations.
But the company has agreed a deal with the US government to take almost its entire stock.
Dr Andrew Hill, a senior visiting research fellow at the University of Liverpool, told Sky News the drug will not be available for use on patients in the UK and Europe until October.
He said: "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.
"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."
Countries with low or middle incomes can produce generic versions of the drug, but are unable to sell them to Europe because Gilead has a patent for it, he added.
Trials of the antiviral medicine on patients have showed it reduces the length of time they experience symptoms from 15 to 11 days by stopping the virus reproducing.
Health Secretary Matt Hancock described the use of remdesivir on COVID patients as the "biggest step forward in the treatment of coronavirus since the crisis began".
However plans to roll it out for patients in UK hospitals will now be on hold.
The US has recorded more than 2.5 million confirmed cases of Covid-19, and President Donald Trump's administration has been widely criticised for its inconsistent approach to tackling the pandemic.
