More than 160 former world leaders and global figures have called on the UK and other rich countries to immediately airlift millions of surplus Covid vaccines to less developed nations, reports BBC.
They say it would be unethical for doses to be wasted while thousands are dying with the virus every day.
It is addressed to Italian PM Mario Draghi, who is hosting the G20 group of major economies in Rome this weekend.
Brown told BBC Breakfast: "Countries have over-ordered and they're over-stocked and they're not giving the vaccines out quickly enough so a lot of vaccines could be wasted past their expiry date."
The letter's signatories include 36 former presidents, 30 ex-prime ministers and another 100 influential global figures.
Among them is the former UN Secretary General Ban Ki-moon, former New Zealand Prime Minister Helen Clark and former Brazilian President Fernando Cardoso.
While more than six billion Covid vaccine doses have been administered worldwide, they say 70 percent of these were administered by only a few countries and just 2 percent of people in low-income countries have received a jab.
Millions more vaccines should be transferred each month, totalling 1.1 billion in the next four months, it adds.
If this is achieved, it says the World Health Organization's target for 70 percent of all adults to be vaccinated by spring next year can be met.
"Without a detailed plan, 100 million vaccine doses will have passed their use-by date at the end of the year," the letter says.
"If we do not act quickly, that figure could exceed 200 million by the end of January 2022."
The UK has pledged to donate more than 100 million doses over the next year, while the US has pledged 500 million doses.
In total, the G7 group of nations promised to donate more than 870 million doses at a summit in the UK in June.
The vaccines will mainly be delivered through the Covax vaccine scheme, which aims to reach the most vulnerable 20 percent of every nation around the world.
Meanwhile, the president of Indonesia, where around 30 percent of the population are fully vaccinated, has also urged richer countries to share their doses.
"Everyone has helped, but in my opinion it's not enough," Joko Widodo told the BBC.
"In this time of crisis, advanced countries need to do more in helping poor countries get vaccines, so that we can overcome this pandemic together."