Britain's largest pharmacy chain stopped taking bookings from under-65s for flu vaccinations because of a high demand for the jab.
Boots said "more people than ever" were requesting vaccinations before the winter flu season.
The surging demand comes after government scientists warned Britons they would face the "double danger" of flu and coronavirus in the cooler months.
Boots, which has about 2,500 outlets across the UK, had already increased its stock of flu jabs by 20 per cent but even that was not enough to meet demand.
“We have seen more people than ever booking early to get their flu vaccinations," a spokeswoman said.
"We have been closely monitoring our stock levels and made the decision this weekend to pause taking new bookings for our private and NHS under-65s flu vaccination services.
“This is to make sure we can vaccinate patients who have already booked their appointments with us.”
The Pharmaceutical Services Negotiating Committee, which supports pharmacies in England, said there was a "massive increase in demand" across the sector for vaccines.
It said pharmacists had been expecting the demand given the experience of Australia and New Zealand over the southern winter.
"The delivery schedules and increased demand for vaccines means some pharmacies report they are temporarily out of stock of some vaccines ahead of their next scheduled delivery and, in some cases, all of their stock has already been administered to patients," a spokeswoman for the committee said.
Well Pharmacies, the third-largest pharmacy chain in the UK, said demand for the flu jab was “the highest we’ve seen”.
"In the first week of our flu service we saw a 68 per cent increase in the flu jab uptake compared to last year," a spokesman said.
When asked whether the pharmacy would begin rationing flu jabs if demand persisted, the spokesman said Well was comfortable with current stock levels.
But he said the chain would keep "a watching brief on stock levels and on any guidance from the NHS on the flu provision service this year".
Public Health England medical director Prof Yvonne Doyle previously said flu kills between 4,000 and 22,000 people in the UK each year.
A study carried out in the first months of the pandemic in the UK showed people who had both flu and Covid-19 were more likely to die than those who only had one infection.
"The last thing you really want to have is flu," Prof Doyle said.
“And if you do think you have either flu or Covid stay at home and self-isolate, that’s to protect yourself – you’d be feeling very miserable – but also to protect others.”
