NHS workers are being targeted by tax dodging schemes promising to save them money.
An investigation by BBC Money Box found that "unscrupulous" umbrella companies were taking a large cut of workers' salaries and putting them in danger of landing a hefty tax bill.
The firms, which exploit "loopholes" in the law, have been targeting social media adverts at key workers drafted in to help with the COVID-19 crisis.
Many of these employees take on work from multiple employers at once and use an umbrella company to legitimately manage their income.
Earnings from different employers are paid to the umbrella company, which then pays tax, national insurance and other deductions on their behalf. After taking a fee, the umbrella company will then pay the worker what is left.
But some rogue firms are taking large commissions in return for hiding a portion of the worker's pay from HMRC.
One advert on Twitter reads: "If you've been drafted in to reinforce the NHS response to the #coronavirus pandemic, we want to assist you."
An undercover Money Box reporter was told it was possible to save thousands of pounds a year legally by hiding a large chunk of a worker's salary from the taxman.
One UK-registered umbrella company said workers could take home 78% of their salary, which is more than they would receive via a standard umbrella company. As a result, a healthcare worker earning £725 ($918.25) per week could take home £60 more.
In return the company would take £80 a week in fees, four times the industry standard, and the government would lose up to half the tax owed.
"It is shocking that unscrupulous promoters of tax avoidance schemes are targeting returning NHS workers during this difficult time. HMRC published [advice] on 30 March warning returning workers about this very issue,” the regulator told Money Box.
"Our advice has always been to steer well clear of such schemes, and to report them to us in confidence for investigation," said HMRC.
If these umbrella companies are challenged by HMRC, workers could end up with crippling bills to pay back the hidden tax, having already lost money in fees, said Judith Freeman, professor of taxation law and policy at the University of Oxford.