The world’s first coronavirus treatment could save thousands of lives across the world in what is being hailed as a “major breakthrough”.

A study of dexamethasone suggests it is the first drug to reduce deaths from coronavirus, and has been described as the most important trial result for Covid-19 so far.

Researchers found the drug reduced deaths by up to a third among patients on ventilators, and by a fifth for those on oxygen.

It has been immediately approved to treat all UK hospitalised Covid-19 patients requiring oxygen, including those on ventilators.

Scientists estimate that if they had known what they now know about dexamethasone at the start of the pandemic, 4,000 to 5,000 lives could have been saved in the UK.

They added that, based on their results, one death would be prevented by treatment of around eight patients on ventilators, or around 25 patients requiring oxygen alone.

This is the only drug that has so far shown to reduce mortality, and it reduces it significantly

The drug could offer hope for hundreds on oxygen support and the 385 in mechanical ventilator beds across the UK.

England’s chief medical officer Professor Chris Whitty said it was “the most important trial result for Covid-19 so far”, and “will save lives around the world”.

Peter Horby, professor of emerging infectious diseases in the Nuffield Department of Medicine, University of Oxford – and one of the chief investigators for the trial, described it as “an extremely welcome result”.

“This is the only drug that has so far shown to reduce mortality, and it reduces it significantly. It is a major breakthrough, I think,” he said.

He told the Downing street press conference that the majority of patients being treated in hospital for coronavirus were likely to benefit from the study into dexamethasone.

Prof Horby added: “What we saw was really quite remarkable.

“In ventilated patients with Covid-19, the drug dexamethasone – so 10 days of treatment with that, which is a tablet or injection – reduces the risk of death by about 35%.

“In patients on the ward who require oxygen and have Covid, it reduces the risk of death by about 20% – that covers about 75% of patients in hospital who will see a mortality benefit from using this drug.”

Martin Landray, professor of medicine and epidemiology at the Nuffield Department of Population Health, University of Oxford, was also one of the chief investigators.

He said: “It’s been around for probably 60 years.

“It costs in the order of £5- £5 for a complete course of treatment in the NHS, and substantially less – probably less than one dollar – in other parts of the world, for example in India.”

The Government’s chief scientific adviser Sir Patrick Vallance said: “This is tremendous news today from the Recovery trial showing that dexamethasone is the first drug to reduce mortality from Covid-19.

“It is particularly exciting as this is an inexpensive, widely available medicine.

“This is a groundbreaking development in our fight against the disease, and the speed at which researchers have progressed finding an effective treatment is truly remarkable.”

Prime Minister Boris Johnson said the dexamethasone study was the “biggest breakthrough yet” in coronavirus treatment.

At the Downing Street press conference he said: “I am proud of these British scientists, backed by UK Government funding, who have led the first, robust clinical trial anywhere in the world to find a coronavirus treatment proven to reduce the risk of death.

“I’m very grateful to the thousands of patients in this country who volunteered for the trials – thank you.”

He said the drug would now be available across the NHS and “we have taken steps to ensure we have enough supplies, even in the event of a second peak”.

Sir Patrick warned that dexamethasone does not stop people catching coronavirus or going into hospital with it.

“It reduces the death for those who are in hospital who require oxygen, or in ICU and require oxygen, and it’s a very important effect,” he said at the briefing.

He also said that now one cheap drug has been shown to work, it is likely that in time combining several of them will also be effective.

The Recovery trial – which stands for Randomised Evaluation of COVid-19 thERapY – was co-ordinated by scientists from the University of Oxford.

The mortality rate of those who end up on a ventilator is above 40%, but this figure was reduced by a third among those prescribed dexamethasone.

More than 11,500 patients from 175 NHS hospitals have been enrolled on to the Recovery trial since it was set up in March to test a range of potential coronavirus treatments.

In the dexamethasone study, 2,104 patients received 6mg of dexamethasone once a day via either mouth or intravenous injection for 10 days.

Their outcomes were compared with a control group of 4,321 patients.

Over a 28-day period, the mortality rate among those requiring ventilation was 41% and 25% among those needing oxygen.

The mortality rate among those not requiring respiratory intervention was 13%.

However, the study did not see any benefit in those patients who were in hospital with Covid-19, but whose lungs were working sufficiently well.

Prof Landray said: “What we can see is the biggest benefits in those people at the biggest risks, which I guess if you wanted to design a drug is exactly how you’d hope to have the results.”

The researchers warn that the steroid has not been studied in patients in the community, and people should not be taking dexamethasone for Covid-19.