The first drug for coronavirus will be rolled out to seriously ill patients, the Government has announced.
From this week, some patients will be given remdesivir to help speed up their recovery.
The drug is currently undergoing clinical trials around the world, including in the UK, with early data showing that it can shorten the time to recovery by about four days.
Matt Hancock, the Health Secretary, said: "This is probably the biggest step forward in the treatment of coronavirus since the crisis began."
The Medicines and Healthcare products Regulatory Agency (MHRA) has cleared the treatment to be used as part of the Early Access to Medicines Scheme (EAMS).
Commenting on the announcement, Dr Stephen Griffin, an associate professor in the School of Medicine at the University of Leeds, said "The news that remdesivir will be available via the EAMS is welcome indeed.
"It will likely mean that the most severe patients will receive it first. It is a welcome boost to the current repertoire of Covid-19 therapies, and it could be important for the longer-term management of the pandemic"
Remdesivir is a broad-spectrum antiviral which was developed by pharmaceutical company Gilead more than a decade ago to cure an unknown "Disease X" amid fears that a global pandemic was looming.
More than a dozen centres across the NHS have been trialling the treatment since April, and the results have been positive.
The drug works by disrupting the genetic code of a virus to prevent it from replicating. Many viruses reproduce in a similar way, so it is hoped the drug will be useful for several different infections.
Until now doctors could not prescribe or use remdesivir outside of trials, but the EAMS scheme aims to give access to promising new unlicensed medicines where there are no other options.
It effectively gives the treatment a form of provisional licence because there are no other drugs which fit the bill. The approval lasts for a year, but can be renewed if it is not ready for a full licence by then.
Dr June Raine, the MHRA chief executive, said: "We are committed to ensuring that patients can have fast access to promising new treatments for Covid-19.
"We will continue to work closely with the Department of Health and Social Care and other healthcare partners on protecting public health in the UK by prioritising our essential work on clinical trials, access to medicines, and the development of vaccines."
Allocation of the drug will be based on expert clinical advice and will take into consideration the situation in which it is most likely to provide the greatest benefit. However, experts said doctors would still need to use caution when prescribing it because trials were ongoing.
Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine, said: "The amount of evidence on remdesivir's efficacy and safety is still limited, and it's possible it will not become licensed.
"It is also not a recommendation for use, but it does allow clinicians to use the medicine outside one of the ongoing randomised trials without any fear of it being regarded as bad practice.
"It probably works better when given early in the course of the disease. It may well reduce mortality, but the totality of evidence so far is not completely convincing on that – but with more trials being conducted around the world, as well as in the UK, such evidence may be obtained soon."
Trials of remdesivir and other treatments will continue, and doctors say it is too early to tell whether the drug will become a standard treatment in the long term.
Lord Bethell, the minister for innovation, said: "The latest, expert scientific advice is at the heart of every decision we make, and we will continue to monitor remdesivir's success in clinical trials across the country to ensure the best results for UK patients."
Trials to find out whether the blood plasma from recovered coronavirus patients could help people who are still sick are being hampered because the disease is dying away, scientists have said.
On Tuesday, researchers in the UK began testing whether convalescent plasma – which contains powerful antibodies against coronavirus – could help speed up recovery. The approach was used during the 1918 Spanish Flu pandemic, before vaccines or antivirals were available, and has recently shown success in Sars and influenza.
But Professor Peter Horby, who is leading the trial, said results may be several months away because it was becoming increasingly difficult to recruit patients.
"The number of patients is really low at the moment, so recruitment will be slow, so it will be some months," he told BBC Radio 4's Today programme. "I'm afraid we will be slower to get an answer."