The UK will share any future surplus coronavirus vaccines with developing countries, Boris Johnson is set to pledge at Friday's G7 meeting - the first time the PM has chaired the event and the first time Joe Biden has attended as US President.
Mr Johnson wants to donate the UK's spare Covid-19 vaccines through global vaccine scheme Covax, rather than attempt to use the life-saving treatments for "diplomatic leverage" with developing nations.
In total, the UK has more than 400 million doses of vaccines on order, enough to vaccinate its population three times over.
But James Cleverly, a Foreign Office minister, said it was "difficult to say with any kind of certainty" when Britain would start to hand out excess jabs, with the country's own vaccine rollout programme still in full flow.
The PM will chair the online gathering of G7 leaders from Downing Street, including US president Joe Biden in his first major multilateral meeting, to discuss the response to the crisis.
Mr Johnson will also urge fellow leaders to back an ambitious target of supporting the development of vaccines for emerging diseases in 100 days in future - a third of the time it took to successfully develop the Pfizer/BioNTech jab.
Mr Johnson said: "The development of viable coronavirus vaccines offers the tantalising prospect of a return to normality, but we must not rest on our laurels."
Former Conservative Party co-chairman Mr Cleverly said Covax, led by the World Health Organisation and other international bodies, had been nominated to receive any surplus supplies in order to avoid diplomatic wrangling.
Speaking to Times Radio, he said: "We've made a very principled and correct decision that the best way of deciding the distribution of the bulk of the vaccine to the world should be done through multilateral bodies rather than using what is potentially a life-saving drug as some form of diplomatic leverage.
"So we've decided to go through Covax which is the international body that will decide the best and most equitable distribution."
Prime Minister Johnson will use the meeting to urge the G7, made up of the US, Japan, Canada, Germany, France and Italy along with the UK, to increase funding for Covax.
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Decisions on timing and the scale of any surplus will be decided later in the year, although Government sources indicate that well over 50% of excess doses would go to Covax.
It will depend on the reliability of the vaccine supply chain and whether new vaccines are needed for emerging variants or booster shots in the autumn.
But with some vaccines that the UK has ordered yet to be approved by the UK regulator, the Medicines & Healthcare products Regulatory Agency (MHRA), Mr Cleverly could not put a date on when the first Covax donations would be made.
The government's current target is to have offered a jab to every adult in the UK by the autumn but, in a sign of the confidence in the current progress rate, the chair of the Vaccines Taskforce Clive Dix said this week that every adult could be given both jabs by August or "maybe sooner".
Asked when the UK would be in a position to share extra vaccines with poorer countries, Mr Cleverly said it was "difficult to say with any kind of certainty".
"There are a number of variables, some of which are in our control," he told BBC Radio 4's Today.
"The speed of vaccinating our own people for example, which is going very well... other variables include when vaccines get the green light by regulators and how quickly the companies can produce those vaccines.
"We're not really able to give with certainty either a timescale or the numbers involved."