Germany has said the Oxford/AstraZeneca vaccine should not be offered to people over the age of 65, a source close to the country's government has told Sky News.
It comes after Reuters reported Germany's vaccine committee made the recommendation, citing insufficient data for it to allow the jab to be rolled out to older age groups.
But UK Prime Minister Boris Johnson said he was not worried because Britain's medicines regulator the MHRA had judged it is "effective across all age groups and provides a good immune response across all age groups".
He added "I don't agree" with the apparent assessment in Germany.
Watch: EU eyes UK vaccine supply as AstraZeneca row deepens
A spokesperson for AstraZeneca said its research findings "support efficacy in the over 65 years age group".
And Public Health England said the immune response data response was also very reassuring, though it added there were too few cases in older people in trials to say what precise level of protection the over 65s get.
The European Medicines Agency is expected to make a decision on whether to approve AstraZeneca's COVID-19 vaccine on Friday.
But Brussels and the pharmaceutical giant are embroiled in an open, escalating row over the EU's slow start to its inoculation programme.
AstraZeneca announced it would have to cut the amount of jabs delivered to the bloc's 27 nations before the end of March from 80 million to 31 million, citing production issues at European factories.
But the EU's health commissioner Stella Kyriakides said AstraZeneca should make up for the shortfall by shipping over jabs made at its plants in Britain instead.
Watch: AstraZeneca to 'cut COVID-19 vaccine delivery to EU by 60%'