Unicef has said this is down to the technical glitches at a South Korean manufacturing plant as well as setbacks in securing export licences from the Indian government.
The Covax programme which is planning to ship coronavirus vaccines worldwide has announced supply delays for up to 90 million doses from an Indian manufacturer.
Covax is a collaboration between Unicef, the World Health Organization (WHO), the vaccine alliance Gavi and the Coalition for Epidemic Preparedness Innovations.
Gavi said the delays come as India is facing a surge of coronavirus infections that will increase domestic demands on the Serum Institute of India (SII), a pivotal vaccine maker behind the Covax programme.
"Delays in securing supplies of SII-produced Covid-19 vaccine doses are due to the increased demand for Covid-19 vaccines in India," Gavi said.
The move will affect up to 40 million doses of the Oxford/AstraZeneca vaccines being manufactured by the Serum Institute that were to be delivered for Covax this month, as well as 50 million expected next month.
The institute has been contracted to supply vaccines to 64 countries, and Gavi said the UN-backed programme has "notified all affected economies of potential delays".
Gavi said the Serum institute has pledged that "alongside supplying India, it will prioritise the Covax multilateral solution for equitable distribution".
Gavi has already distributed 31 million doses of the AstraZeneca vaccine, 28 million from the Serum Institute and another three million from a South Korean contractor also producing the vaccine.
The programme had been aiming to deliver some 237 million AstraZeneca vaccines through the end of May.
A Gavi spokesperson said the delays were not expected to affect the goal of shipping some two billion doses worldwide through Covax by the end of the year.
Covax has so far shipped vaccines to some 50 countries and territories.
UN officials, governments, advocacy groups and others have pleaded with manufacturers to do more to speed up and widen production of Covid-19 vaccines and ensure fair distribution - insisting that the pandemic can only be defeated if everyone is safe from it.