Details will be announced ahead of the prime minister meeting world leaders – some of whom have made donations – at the G7 summit this week, No 10 said.
Joe Biden has already announced that the US will send 25 million doses overseas, three-quarters through the Covax aid programme and the rest directly to most-in-need countries.
Emmanuel Macron also said France would send 500,000 shots, with some already on their way to West Africa – making two leaders that Mr Johnson will meet face-to-face at the summit in Cornwall.
In contrast, the UK has said only that it will donate some “surplus doses”, with no hint of how many will be released or when, given the domestic vaccination programme must be completed first.
The UK has secured access to around 400 million jabs – far more than it is likely to need – but autumn booster shots are now planned and vaccinations of children are increasingly likely.
Downing Street said the UK had “already pledged to share a significant majority of its surplus doses” with the Covax scheme, to which it is the largest donor.
“Later this week, the prime minister will announce more details of the UK’s plans to support developing countries by sharing doses not needed by the UK,” a statement said.
It is unclear how specific Mr Johnson will be, after a group of 100 MPs and peers urged him to commit to matching every dose bought for the UK with a foreign donation.
The UK argues it has provided global leadership, by funding the Oxford-AstraZeneca vaccine and making it available at cost – rather than at a much higher price, with huge profits made.
Almost 1 in 3 vaccines administered around the world have been the Oxford jab – 450 million out of 1.5 billion doses – although some countries prefer the Pfizer-BioNTech version.
Mr Johnson will counter criticism by calling for “concrete commitments” from fellow G7 leaders to vaccinate “the entire world” by the end of 2022.
“The world is looking to us to rise to the greatest challenge of the post-war era: defeating Covid and leading a global recovery driven by our shared values,” the prime minister said.
“Vaccinating the world by the end of next year would be the single greatest feat in medical history.
“I’m calling on my fellow G7 leaders to join us to end to this terrible pandemic and pledge will we never allow the devastation wreaked by coronavirus to happen again.”