Millions of Covid vaccines could be wasted if rich countries send large amounts of leftover doses to poorer nations in one go, UNICEF has warned.
The charity said there needed to be a steady supply throughout the year because poor countries do not have resources to use them all at once.
The UK and others have promised to donate their surplus doses – but they have been asked to give 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% 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.
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 Mr Hancock said the UK did not currently have any spare doses.
