Actors Liam Neeson, Orlando Bloom, Gemma Chan, and Whoopi Goldberg are also among stars to sign their support as well as singers Katy Perry and Pink, and UK tennis ace Andy Murray.
Unicef warned that the world would continue to be a risk from future Covid-19 variants if “fair and equitable” vaccine supplies are not ensured worldwide.
Former England footballer David Beckham said: “The pandemic won’t be over until it’s over everywhere, so it’s vital that all communities around the world have fair access to Covid-19 vaccines urgently.”
The letter has been signed by 28 high-profile supporters from around the world.
“The world has spent a year and a half battling the Covid-19 pandemic but the virus is still spreading in many countries and producing new variants with the potential to put us all back where we started,” the letter reads.
“This means more school closures, more healthcare disruptions and greater economic fallout - threatening the futures of families and children everywhere.
“This weekend’s G7 Summit is a vital opportunity for you to agree the actions that will get vaccines where they are most needed, fast.”
The letter also warned that Covax, the global initiative supporting poorer countries in gaining access to vaccines, is facing a shortfall of 190 million doses.
Foreign Secretary Dominic Raab previously pledged that poor countries must have “fair access to vaccines” and said Britain will take a lead to ensure it happens.
Speaking to the Evening Standard earlier this year, he said the UK will put “global health at the forefront” when Boris Johnson hosts the G7 summit in Cornwall of world leaders.
Meanwhile Unicef has proposed that G7 countries should donate 20% of their vaccines between June and August to help address the shortages temporarily.
Unicef executive director, Henrietta Fore, added: “Countries need not choose between fighting the disease at home or fighting it abroad.
“We can, and must, do both simultaneously - and immediately.
“This is a pivotal time in the fight against Covid-19, as leaders meet to set priorities for what form this fight will take in the coming weeks and months.
“After all, the disease is not respecting boundaries on a map. Our fight to get ahead of the virus, and its variants, should not either.”