Senior doctors are calling on England's chief medical officer to cut the gap between the first and second doses of the Pfizer-BioNTech Covid-19 vaccine.
Prof Chris Whitty said extending the maximum wait from three to 12 weeks was a "public health decision" to get the first jab to more people across the UK.
But the British Medical Association said that was "difficult to justify" and should be changed to six weeks.
It comes as early evidence suggests the UK virus variant may be more deadly.
Prime Minister Boris Johnson told a Downing Street briefing on Friday: "In addition to spreading more quickly, it also now appears that there is some evidence that the new variant - the variant that was first identified in London and the south east - may be associated with a higher degree of mortality."
Previous work suggests the new variant spreads between 30% and 70% faster than others, and there are hints it is about 30% more deadly.
For example, the government's chief scientific adviser Sir Patrick Vallance said if 1,000 men in their 60s were infected with the old variant, roughly 10 of them would be expected to die - but this rises to about 13 with the new variant.
Another 1,348 deaths within 28 days of a positive coronavirus test were reported in the UK on Saturday, in addition to 33,552 new infections, according to the government's coronavirus dashboard.
The government's Joint Committee on Vaccination and Immunisation (JCVI) says unpublished data suggests the Oxford-AstraZeneca vaccine is still effective with doses 12 weeks apart - but Pfizer has said it has tested its vaccine's efficacy only when the two doses were given up to 21 days apart.
The World Health Organization has recommended a gap of four weeks between doses - to be extended only in exceptional circumstances to six weeks.
Government minister Robert Jenrick said the current strategy ensured "millions more people can get the first jab" and the "high level of protection" which it offered.
He said the BMA's concerns would be taken into account but that the government was following the "very clear advice" of the medicines regulator and the UK's four chief medical officers who, he said, "could not have been clearer that this is the right thing to do for this country".
A spokeswoman for the Department of Health and Social Care added: "Our number one priority is to give protection against coronavirus to as many vulnerable people as possible, as quickly as possible."
'Deep concern' over UK approach
In the letter to Prof Whitty, seen by the BBC, the British Medical Association (BMA) said it agreed that the vaccine should be rolled out "as quickly as possible" - but called for an urgent review and for the gap to be reduced.
The doctors' union said the UK's strategy "has become increasingly isolated internationally" and "is proving evermore difficult to justify".
"The absence of any international support for the UK's approach is a cause of deep concern and risks undermining public and the profession's trust in the vaccination programme," the letter said.
Dr Chaand Nagpaul, chair of the BMA, said there were "growing concerns" that the vaccine could become less effective with doses 12 weeks apart.
"Obviously the protection will not vanish after six weeks, but what we do not know is what level of protection will be offered [after that point]," he told BBC Breakfast.
"We should not be extrapolating data when we don't have it."
He said while he understands the rationale behind the decision, "no other nation has adopted the UK's approach".
"We think the flexibility that the WHO offers of extending to 42 days is being stretched far too much to go from six weeks right through to 12 weeks," he added.
There has been understandable enthusiasm over a promising start to the hugely ambitious UK vaccination rollout.
But there has been some tension over the decision to lengthen the time between doses for the Pfizer vaccine to 12 weeks.
Prof Whitty and other health leaders and experts say this will allow many more people to get vaccinated quickly and the first dose gives most of the protection.
But critics argue this goes against Pfizer's recommendation of a three-week gap and there is no data to back up the long delay.
The intervention of the BMA is significant as it shows senior doctors now have widespread concerns, including worries about reliability of supplies if people have to wait longer for a second jab.
This is a private letter to Chris Whitty seen by the BBC and not a grandstanding press release. The BMA wants to have talks with the chief medical adviser about moving to six weeks.
Prof Whitty will no doubt restate his case, but it will be interesting to see whether the BMA argument gains traction in the wider medical world.
The BMA also suggested second doses might not be guaranteed after a 12-week delay "given the unpredictability of supplies".
However, Public Health England's medical director said people would get their second dose.
Dr Yvonne Doyle told BBC Radio 4's Today programme that she backed the current strategy, saying it was "about bearing down on transmission" to reduce deaths and reduce the chance of more dangerous variants of the virus emerging.
"The more people that are protected against this virus, the less opportunity it has to get the upper hand," she said.
Other issues highlighted in the letter include:
- concerns that one dose of the Pfizer jab "does not produce sufficient neutralising antibodies and the potential to reduce transmission"
- a mismatch between policies across the UK - in particular, Northern Ireland's decision to give the second Pfizer vaccine to care home residents and staff within three weeks
- reports from Israel's vaccination programme suggesting "lower protection than expected"
The UK's chief medical officers have said the "great majority" of initial protection comes from the first jab, while the second dose is likely to help that protection last longer.
In total, the UK has ordered 100 million doses of the Oxford-AstraZeneca vaccine and 40 million of the Pfizer vaccine.
Both vaccines are expected to work against the variant of Covid-19 that emerged in the UK.
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