There are "heartbreaking" cases of employees being made redundant after being denied help from the Government's job retention scheme, Citizens Advice has said.
The charity fears the Government's action to protect workers during the coronavirus pandemic could be undermined by confusion and "patchy use" of the scheme by some employers.
Advisers are seeing daily cases of people who are at risk of slipping through the safety net despite potentially being entitled to support.
Research by the charity shows around six million people in the UK have had their hours cut, been laid off or made redundant.
Almost four out of 10 have lost household income because of the crisis, with nearly one in 12 losing 80% of household income.
Demand for the charity's redundancy advice has doubled, with its web page titled "what to do if your employer has told you not to work" being the most-read page this month with more than 175,000 views.
Dame Gillian Guy, chief executive of Citizens Advice, said: "The Government has unveiled a remarkable package of support to help people keep their heads above water during this crisis.
"It must now ensure employers are using this to protect their workers, as well as opening it up to cover even more people.
"We're already starting to see heartbreaking cases where employees have been denied help from the scheme and have instead been made redundant.
"While it's not clear how much of this is due to confusion, cash flow fears, or the need to downsize, we think the Government needs a clear plan to protect people if the support is not filtering through.
"That is the only way to reduce the burden on the benefits system, and retain the link between an employer and their staff."
Earlier this month, the Government offered to help pay wages to try to stop people losing their jobs due to the Covid-19 pandemic.
The Coronavirus Job Retention scheme gives people 80% of their usual earnings, meaning they can be furloughed rather than laid off from their place of work.
Citizens Advice highlighted the case of a father-of-four called Mark who returned to work as an HGV driver in February, after spending eight months caring for his daughter who is undergoing treatment for a rare cancer.
In March, the family were told they needed to self-isolate as his daughter is one of the 1.5 million who must be shielded.
Mark was made redundant the next day as he could not return to work.
He said: "I spoke to my manager and explained that if someone in my home caught coronavirus and passed it on to my daughter the chances are she would die.
"They said that as a driver I would not be in contact with many people and could I not just self-isolate in my cabin. I said that wouldn't be possible.
"My family and their wellbeing comes first, so I have to accept that I no longer have a job.
"It's just really insensitive and feels wrong that they can let me go for following the Government guidelines to shield my daughter as she is high risk."
Citizens Advice is also calling on the Government to look again at those who are not yet covered by the job retention scheme.
The charity said they includes people at higher risk of coronavirus, such as those who are pregnant or have diabetes, but who are unable to work from home.
Citizens Advice is also concerned about those who have recently become self-employed but are not covered by the Self-Employed Income Support Scheme.
Daniel Heath, 29, from Haverhill near Cambridge, is manager at Nuclear High Ropes in Essex, and said he cannot be furloughed as he started the job after the Government's cut-off date of February 28.
He told the PA news agency: "My frustration is with the Government, definitely.
"My current employer, or my new employer, definitely wants to furlough me, and they want to help, but they can't because of the restriction they put in place of the 28th of February cut-off.
"And the Government keep saying to go back to your previous employer, but my previous employer won't entertain it."
Mr Heath, who is an NHS volunteer, is among almost 7,000 members of a Facebook group calling for support for workers who changed job after February 28.
"We're all in that position where we need it to happen," he said, adding: "I'm now stuck with all these bills, a new house that we've just bought, and no way of paying for it.
"My partner's got an income, but it's not going to be enough to fund two people and pay for all the bills, and the food and the mortgage and the car and everything."
Mr Heath said he is being almost forced to break the Government guidelines about staying at home in order to go out looking for work.
"It's putting myself at risk, and many others, I'd imagine, because we're trying to search for a job to fill the gap," he said.