Dr Bruce Aylward, senior leader at the WHO, said it meant the Covid crisis could "easily drag on deep into 2022".
Less than 5% of Africa's population have been vaccinated, compared to 40% on most other continents.
The UK has delivered more than 10 million vaccines to countries in need.
It has pledged a total of 100 million.
Dr Aylward appealed to wealthy countries to give up their places in the queue for vaccines in order that pharmaceutical companies can prioritise the lowest-income countries instead.
He said wealthy countries needed to "stocktake" where they were with their donation commitments made at summits such as the G7 meeting in St Ives this summer.
"I can tell you we're not on track" he said. "We really need to speed it up or you know what? This pandemic is going to go on for a year longer than it needs to."
The People's Vaccine - an alliance of charities - has released new figures suggesting just one in seven of the doses promised by pharmaceutical companies and wealthy countries are actually reaching their destinations in poorer countries.
The vast majority of Covid vaccines have been given in high-income or upper middle-income countries. Africa accounts for just 2.6% of doses administered globally.
The group of charities, which includes Oxfam and UNAids, also criticised Canada and the UK for procuring vaccines for their own populations via Covax, the UN-backed global programme to distribute vaccines fairly.