The government has ended shielding advice meaning the most vulnerable during the coronavirus outbreak can now leave their home and go to work.
Some 2.2 million people with underlying severe health conditions were advised to stay at home and avoid non-essential face-to-face contact under the guidance.
Around 595,000 (28%) of those usually work, according to charities.
Shielding advice has now ended in England, Scotland and Northern Ireland.
The move comes as the government postponed the easing of certain lockdown measures with some businesses that were hoping to reopen being told they must remain closed for now.
Bowling alleys, casinos, skating rinks and beauty salons offering close-contact services like facials had been scheduled to welcome customers today for the first time since lockdown, while small wedding receptions and indoor performances were set to resume.
Their doors will stay shut for at least two more weeks after an increase in COVID-19 cases in England.
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A coalition of charities is urging Chancellor Rishi Sunak to protect the jobs of workers who followed the advice, warning they will be put in an "impossible position" now that restrictions are eased.
An open letter signed by 15 charities, including Age UK and Macmillan Cancer Support, raises concerns that those who have been shielding will be forced to choose between their health and their job.
The signatories have warned these employees are at risk of being made redundant, or could be forced to return to the workplace when they do not feel it is safe to do so.
The letter to Mr Sunak says: "Our concern is that, especially as your furlough arrangements start to unwind and the shielding scheme is paused from next week, some of these workers will find themselves in an impossible position.
"This is because if their occupation is one which they cannot carry out from home, and if it is extremely difficult to make their workplace safe for them, they may be forced to choose between putting their health on the line by returning, or staying safe by giving up their job."
The signatories say this is "desperately unfair" for those who have made "great sacrifices" by staying at home, and call on the chancellor to take action and protect their jobs as well as supporting employers.
This could include extending the furlough scheme for those who have been shielding or are at high risk, the letter adds.
The same suggestion has been made by the TUC, with General Secretary Frances O'Grady telling the Guardian: "It would be heartless and reckless for employers to demand the immediate return of shielding workers.
"After self-isolation for a number of months, requiring shielding workers to immediately travel to workplaces may cause anxiety.
"The job retention scheme is in place until at least October, so employers must continue using it if home working is not an option."
A survey conducted by Macmillan Cancer Support with 2,000 adults found many cancer patients are fearful of returning to workplaces, with 42% saying they feel it is currently unsafe for them to work outside of their home.