Doctors in Britain urged the government to halve the 12-week gap between giving people the first and second shots of the Pfizer-BioNTech Covid-19 vaccine.
Current policy is focused on administering a first dose of the vaccine to as many people as possible to give some initial protection.
This is despite Pfizer and BioNTech issuing a warning that there was no evidence their vaccine would offer protection against the virus if the second dose was given more than three weeks later.
The British Medical Association, a trade union for doctors and medical students in the UK, asked England's chief medical officer, Prof Chris Whitty, to "urgently review" the policy.
"The UK's strategy has become increasingly isolated from many other countries," the association said.
“BMA members are also concerned that, given the unpredictability of supplies, there may not be any guarantees that second doses of the Pfizer vaccine will be available in 12 weeks’ time.”
Yvonne Doyle, medical director of Public Health England, defended the policy as "a reasonable scientific balance on the basis of both supply and also protecting the most people".
The UK is also using the vaccine developed by AstraZeneca, which supported the gap between doses.
On Saturday, Britain had recorded 97,329 deaths among people who tested positive for the virus, the highest number of fatalities in Europe.