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 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% of their vaccines by August.
“The pandemic will not be over anywhere until it is over everywhere,” said Beckham.
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 vaccinate 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 Mr Hancock said the UK did not currently have any spare doses.
Some countries have pledged doses by the end of the year – but the World Health Organization has asked nations to share their vaccines now.
At the G7 summit later this week, Prime Minister Boris Johnson is planning to urge setting a target to vaccinate the world by the end of 2022.
Campaigners and critics – as well as a group of MPs – say action needs to be taken now and Britain should donate a dose for every jab imported into the UK.
The UK government has already donated £548m in aid money to the Covax scheme, and half a billion doses of the Oxford/AstraZeneca jab have been ordered by other countries and sent abroad.
In a statement, the government said: “The UK has played a leading role in championing global access to coronavirus vaccines.
The PM has said the UK would share the majority of any future surplus vaccines with the Covax pool when they are available and he will call on fellow G7 leaders to make concrete commitments to vaccinate the entire world against coronavirus by the end of 2022 during the G7 summit later this week.
“So far, we’ve not only provided £548m to help the Covax scheme deliver more than a billion vaccines to lower-middle income countries this year, but also over half a billion doses of the government-backed Oxford/AstraZeneca vaccine have been provided around the world on a non-profit basis.”
The leaders of the seven G7 countries are meeting in Cornwall for their annual summit this weekend. The G7 countries are the UK, Canada, France, Germany, Italy, Japan and the US.
The US, France, Germany, Italy and Japan have all said how many doses they will donate to Covax. Like the UK, Canada is yet to put a figure on their planned contribution