It said the drug was part of the world's biggest trial testing existing treatments to see if they also work for coronavirus.

It said the drug was already being used to reduce inflammation in a range of other conditions, and it appeared to help stop some of the damage that could happen when the body's immune system goes into overdrive as it tries to fight off coronavirus.

The body's over-reaction is called a cytokine storm and it can be deadly.

In the trial, led by a team from Oxford University, around 2,000 hospital patients were given dexamethasone and were compared with nearly 4,000 who did not get the drug.

For patients on ventilators it cut death risk from 40% to 28%. For patients needing oxygen it cut death risk from 25% to 20%.

Chief investigator Prof Peter Horby said: "This is the only drug so far that has been shown to reduce mortality and it reduces it significantly. It's a major breakthrough."

Lead researcher Prof Martin Landray says the findings suggest that for every eight patients needing ventilators that you treat, you could save one life.

In patients on oxygen, you save one life for every 20-25 or so treated with the drug.

"There is a clear, clear benefit. The treatment is up to 10 days of dexamethasone and it costs about £5 per patient. So essentially, it costs £35 to save a life. This is a drug that is globally available."

Prof Landray said, when appropriate, hospital patients should now be given it without delay, but people should not go out and buy it to take at home.

Dexamethasone does not appear to help people with milder symptoms of coronavirus - people who don't need help with their breathing.

The Recovery Trial has been running since March and included the malaria drug hydroxychloroquine that has now been ditched amid concerns that it increases fatalities and heart problems.

Another drug called remdesivir, an antiviral treatment that appears to shorten recovery time for people with coronavirus, is already being made available on the NHS.