Germany's vaccine committee has said AstraZeneca's Covid jab should only be given to people aged under 65.
The committee cited "insufficient data" over its efficacy for older people.
The European Medicines Agency is to decide on Friday whether to approve the vaccine for use across the EU.
The UK has been using the AstraZeneca vaccine in its mass immunisation programme for weeks now, and public health officials say it is safe and provides "high levels of protection".
The German announcement comes as the EU is in dispute with leading manufacturers over a shortage of vaccines on the continent.
UK-based AstraZeneca has said production issues at its Europe-based plants mean it will be unable to deliver the promised number of doses to the bloc.
But the EU says the firm must honour its commitments and deliver the jabs by diverting stock from the UK. It is calling for "robust action" to secure the supply.
Pfizer-BioNTech has also cut the number of doses it is delivering to the 27-member bloc.
UK confident 'safe' vaccine has 'substantial benefits'
The independent vaccine commission advising the German government said on Thursday that there were "currently insufficient data available to assess the vaccine efficacy from 65 years of age" and recommended "the AstraZeneca vaccine... should only be offered to people aged 18-64 years at each stage".
But Dr Mary Ramsay, Head of Immunisations at Public Health England, said both the AstraZeneca and Pfizer-BioNTech vaccines are "safe and provide high levels of protection against Covid-19, particularly against severe disease.
"There were too few cases in older people in the AstraZeneca trials to observe precise levels of protection in this group, but data on immune responses were very reassuring."
UK Prime Minister Boris Johnson said he was not worried by the German recommendation.
"Our own authorities have made it very clear that they think the Oxford-AstraZeneca vaccine is very good and efficacious, gives a high degree of protection after just one dose (...) and the evidence that they've supplied is that they think it is effective across all age groups," he said.
Meanwhile, Paul Hunter, professor of Medicine at the University of East Anglia, told BBC News that the elderly should not worry about receiving the jab: "We do know that it is safe in people over 65. They have much fewer side effects than younger people and it almost certainly provides substantial benefits in terms of preventing severe disease and reduce the chances of going into hospital."
What's happening with the rollout in the EU?
The pace of the EU's vaccination programme has been criticised in recent weeks, with Germany and other members failing to hit their own targets. Officials have blamed a combination of logistical and supply issues.
In Spain, authorities in the capital Madrid have suspended all first doses of the vaccination for at least two weeks. Other regions, including Catalonia, are having similar problems with stock.
There are similar shortages due to delays in shipments of both the Pfizer-BioNTech and Moderna vaccines, which are the only ones currently approved for use in the EU.
European Council President Charles Michel said in a letter to four EU leaders: "If no satisfactory solution can be found, I believe we should explore all options and make use of all legal means and enforcement measures at our disposal."
Portugal is recording a high number of new infections and reports suggest hospitals in Lisbon are struggling to cope with the influx of patients.
In France, health authorities are delaying vaccination appointments. Cases have increased and President Emmanuel Macron is reportedly considering a stricter lockdown.
What's behind the supply problems?
The EU signed a deal with AstraZeneca in August for 300 million doses, with an option for 100 million more, but the UK-Swedish company has reported production delays at plants in the Netherlands and Belgium.
AstraZeneca CEO Pascal Soriot said production was "basically two months behind where we wanted to be".
The EU had hoped that as soon as approval was given delivery would start straight away, with some 80 million doses arriving in the 27 nations by March. But the production issues have dented this hope.
Reports said last week the EU would get 60 percent fewer vaccine doses - about 50 million jabs - than promised in the first quarter of the year.
The bloc is also facing delays with supplies of the Pfizer-BioNTech vaccine, and it has a much bigger deal with the US-German vaccine maker.
Both the EU and AstraZeneca have vowed to work together to resolve the problems, and crisis talks were held on Wednesday.