UNICEF warned that millions of Covid vaccines could be wasted if rich countries send large amounts of leftover doses to poorer nations in one go.

It said that supply to poor countries should be steady throughout the year because they do not have resources to use them all at once.

The UK and others have pledged donating their surplus doses – but they have been asked to give more earlier. Stars including Billie Eilish and David Beckham are backing Unicef’s plea.

The celebrities have signed a letter to the G7 group of rich countries – including the UK – asking them to donate 20 percent of their vaccines by August.

The other stars who have signed the letter include Andy Murray, Olivia Colman, Ewan McGregor, Liam Payne, Priyanka Chopra Jonas, Orlando Bloom, Katy Perry, Gemma Chan, Whoopi Goldberg, Claudia Schiffer and Chris Hoy.

Unicef’s vaccine lead Lily Caprani told BBC Newsnight that countries needed to inoculate their own populations at the same time as the rest of the world.

“At some point, no doubt, we will need to vaccinate under-18s,” she said. “But the priority at this moment has to be making sure that all of the vulnerable and priority groups around the world get vaccines.

“So we’re saying countries like the UK and the G7 need to donate their doses to those low income countries now, while still vaccinating their populations at home.”

But last week, Health Secretary Matt Hancock said vaccinating children in the UK would take priority over sending doses abroad.

Unlike other countries, the UK has not revealed how many doses it plans to donate to the Covax vaccine-sharing scheme, saying only that it will donate its excess doses. On Friday Hancock said the UK did not currently have any spare doses.