Nov 4 (Reuters) - Britain on Thursday became the first country in the world to approve a potentially game-changing COVID-19 antiviral pill jointly developed by Merck and Ridgeback Biotherapeutics, in a boost to the fight against the pandemic.

The Medicines and Healthcare products Regulatory Agency (MHRA) recommended the drug, molnupiravir, be used as soon as possible following a positive COVID-19 test and within five days of the onset of symptoms.

This is the first oral antiviral treatment for COVID-19 to get approved, with the green light coming ahead of potential U.S. regulatory clearance. U.S. advisers will meet this month to vote on whether molnupiravir should be authorized.

The drug, to be branded Lagevrio in Britain, has been closely watched since data last month showed it could halve the chances of dying or being hospitalised for those most at risk of developing severe COVID-19 when given early in the illness.

The British government and the country's National Health Service will confirm how the treatment will be deployed to patients in due course.

Last month, Britain agreed a deal with Merck to secure 480,000 courses of molnupiravir.

In a separate statement, Merck said it was expecting to produce 10 million courses of the treatment by the end of this year, with at least 20 million set to be manufactured in 2022.

The U.S. based drugmaker's shares were up 2.1% at $90.54 before the market open.