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Boris Johnson has said it would be “peculiar” to ban the export of a drug that can be used to save the lives of coronavirus patients, hours after the Department for Health had done just that.

On Tuesday the government announced a study of dexamethasone suggests it is the first drug to reduce deaths from the illness.

Speaking at the Downing Street press conference, the prime minister hailed it as “biggest breakthrough yet” in the fight against the pandemic.

But when asked why dexamethasone had been added to a list of drugs that could not be exported from the UK, Johnson said he was “not aware” it was.

Johnson added such a ban “sounds peculiar to me” and he would “look into it”.

According to a government list of medicines that cannot be exported from the UK or hoarded, dexamethasone was banned for export in tablet form on April 24, and as an oral solution or for injection on Tuesday.

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.

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.

“Dexamethasone is inexpensive, on the shelf, and can be used immediately to save lives worldwide.”

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.