LONDON: Britain said on Thursday that there were lumps and bumps in the global vaccine supply chain that were causing slower deliveries than expected and scolded the European Union for threatening to slap a ban on vaccine exports.

British health officials cautioned the health service on Wednesday that there would be a significant reduction in vaccine supplies from March 29 though Pfizer Inc and AstraZeneca Plc said their delivery schedules had not been impacted.

"We always said right from the beginning that a new manufacturing process would have its lumps and bumps and that has been the case in the past and I'm sure it will be in the future," Housing Secretary Robert Jenrick told Sky.

"We're sourcing vaccines from all over the world and we are experiencing occasionally some issues and that's led to this, this issue with some supply in the coming weeks," he said.

Britain is on track to have given a first COVID-19 shot to half of all adults in the next few days, making it one of the fastest countries in the world to roll out a vaccine.

Asked if the issue was supply from India, Jenrick said he did not think it was right to discuss specific contracts.

He said the country remained on track to have vaccinated priority groups by April 15 and all adults by the end of July.

"The main thing is, we're still very much on course, we've still got line of sight to deliver the vaccines and to meet our targets," Jenrick said.

The European Union threatened on Wednesday to ban exports of COVID-19 vaccines to Britain to safeguard scarce doses for its own citizens facing a third wave of the pandemic that would jeopardise plans to restart travel this summer.

The threat from European Commission head Ursula von der Leyen was disappointing, Jenrick said.

"I was surprised and disappointed by those comments but the prime minister had spoken earlier in the year to Ursula von der Leyen and she gave a very clear commitment, which was that the EU would not engage in this sort of activity, that contractual responsibilities would be honoured.

"And that's exactly what we intend to do and I hope and expect the EU to stick to their side of the bargain."