Former UK leader Gordon Brown has said the world will need 10 billion Covid-19 doses to vaccinate the entire global population.
Mr Brown was speaking after UK Prime Minister Boris Johnson urged world leaders to commit to vaccinating the world against coronavirus by the end of next year.
Mr Johnson's request came ahead of Britain hosting the G7 summit in Cornwall this week.
Asked about the prime minister’s initiative, Mr Brown said on Monday: “He’s not doing enough – this is a promise, not a plan.
“What he’s proposing is to share doses, so our excess doses will go somewhere else, but that would only raise about 500 million vaccines.
"We need 10 billion vaccines to be provided to vaccinate the rest of the world.”
Mr Brown proposed an agreement to share the burden, whereby rich countries help to build the vaccination campaigns of countries in the developing world.
He has previously suggested that based on the national income of G7 nations, the US should pay 27 per cent of the cost of vaccinating the populations of poor nations; Europe 23 per cent; Japan 6 per cent; Britain 5 per cent; and Canada – plus South Korea and Australia, also attending the G7 – 2 per cent each.
“We have manufacturing happening on every continent and we get everybody vaccinated – it’s pretty simple and it’s got to be done,” Mr Brown said.
“You can’t do it by dose-sharing, which is just the leftovers from Britain going somewhere else.”
The Covax initiative, the programme delivering doses to poor countries, has been crippled by supply shortfalls and a gap in funding.
The World Health Organisation estimated last week that Covax was about 200 million doses behind schedule.
The shortfall comes after India stopped exports from its Serum Institute, the world's largest vaccine maker by volume, to deal with the Covid surge there.
While recent pledges to donate 150 million doses through Covax represent a "great start", Bruce Aylward from the WHO's Covax team said there were still "two big problems".
"Number one, very little is committed to the June-July period, which means we're going to still have this gap," Mr Aylward said last week.
"The other problem is just the volume. If we are going to get on track to get at least 30 to 40 per cent of the world population vaccinated this year, we’ve got to get another 250 million people vaccinated between now and the end of September."
Covax’s original objectives were to deliver two billion doses of vaccines worldwide in 2021, and 1.8 billion doses to 92 lower-income countries by early 2022.
The initiative has so far shipped just 80 million doses, according to its June 4 update.