“Millions of people will go unvaccinated and thousands of people, I’m afraid, will die,” the former prime minister said, after the gathering failed to agree a financing package.
On the eve of the summit, more than 100 former world leaders called for the G7 to pay two-thirds of the $66bn cost of a comprehensive global vaccination programme.
But the UK failed to even put it on the agenda, with the G7 instead agreeing merely to donate only 870 million of the 11 billion doses needed across the world.
Dominic Raab claimed the package would enable the world to be vaccinated by the middle of next year, but Mr Brown said that would not now be possible.
“We will have a huge problem of a division between the richest countries that are safe and the poorest countries that are not safe,” he warned, on Sky News.
“But then the problem will come back to haunt the richest countries, because we will have contagions spreading that may hurt even the people who are vaccinated, because of mutations and variants.”
Hopes were raised for the summit when Boris Johnson demanded “concrete commitments” from fellow G7 leaders to secure a plan to vaccinate “the entire world” by the end of 2022.
The former world leaders urged the rich nations to pay two-thirds of the bill for jabs in poor countries, to trigger the bonus of a $9 trillion economic bounce back by 2025, the International Monetary Fund said.
“I wait to see the final communique, but it looks as if there is a huge gap between what Boris Johnson promised last Sunday and what is actually being delivered,” Mr Brown said.
He blamed the UK’s savage overseas aid cuts, explaining: “We’re not prepared to put up enough money to deal with problems that the world faces, even when the benefits in extra economic activity if the world gets back to growth and trade far outweigh the costs of doing this.
“And so Boris Johnson really has got to think, ‘are we to leave this problem to the G20 next year or the year after’, because – on the basis of the evidence, I see – the whole world will not be vaccinated by the middle of 2022.
“It will take months before the world gets back to discussing this again in the most coherent way. We need a plan – we don’t have yet a plan.”
On Friday, Save the Children said it was “deeply disappointing that Boris Johnson is avoiding the main issue he faces during this G7 summit: finding the funds to vaccinate the world”.
And the group Global Justice Now, said: “Boris Johnson’s lofty promises to vaccinate the world have today been wiped out like a surfer in Carbis Bay. It’s shameful.”