Employers are being told to give staff paid time off work to get their coronavirus vaccinations.
More than 75% of adults have now received two Covid jabs, the government has said.
A total of 39,688,566 people have now received both doses, while another 7 million have only had a single dose, according to the latest figures.
Boris Johnson described the milestone in the vaccine rollout as a "huge national achievement".
But figures show one in four UK companies is not giving staff paid time off to get their jabs - and nor do they have any plans to allow this.
Employment rights advocates say firms need to do their bit to protect staff.
Have you been forced to choose between getting a vaccine and going to work? Get in touch: emma.munbodh@mirror.co.uk
Susan Clews, Acas chief executive, said: "The vaccine rollout programme has gone well and our survey reveals that most employers have allowed staff paid time off to get the jab, but a quarter have not.
"Our study also reveals that a similar number of bosses have not paid full company sick pay to staff who have been unable to work due to side effects from the vaccine.
"It's in businesses' best interests to have a vaccine policy that supports staff to take time off, as fully vaccinated workers are less likely to need longer periods of time off work to recover from COVID-19."
The survey of 2,000 businesses found that 59% were giving staff paid time off for vaccinations, 4% were not but plan to implement this in the near future, 25% said they were not and do not plan to, and 12% said they did not know.
It comes as the country prepares for the winter boost jab.
Andy McDonald, shadow employment rights and protections secretary, said: "Employers who are refusing to give their staff paid time off to get their vaccinations are putting their staff at risk and undermining the national effort against the virus.
"The government is not doing enough. They should put pressure on employers to guarantee that workers will receive paid time off for their vaccination appointments and to recover from any after effects."
The Confederation of British Industry's director of policy John Foster said: "Delivering on the vaccine rollout is crucial to boosting confidence in the reopening and helping the economy adjust to the next phase of learning to live with the virus.
"The vast majority of businesses are continuing their commitment to protecting staff and customers during the pandemic. This includes showing flexibility when the time comes for their staff to get the jab.
"We'd encourage all companies to demonstrate this same level of consideration towards their employees."
