The government has been urged to expand eligibility for self-isolation support grants in England.
Currently, people asked to stay at home by the NHS Covid app do not qualify for the £500 grant given to people on low incomes who are told to self-isolate by NHS Test and Trace.
Parents who have to stay at home to look after children who are self-isolating are also excluded.
Citizens Advice said it means people face an “impossible choice”.
People face having to take a “big hit” to their income and may end up struggling with bills in order to help stop the spread of the virus, the charity said.
The government said it was exploring how to expand the system to cover those advised to isolate by the app.
The support scheme is designed to help people who cannot work from home and lose income because they are self-isolating.
If you test positive, you are provided with a code, which you can use to apply for the grant.
But a number of loopholes have emerged – which charities say need to be addressed.
As well as parents looking after isolating children and those pinged by the app, people who are not on benefits are not automatically eligible. They have to apply for discretionary funding instead.
Charities have said that could mean people who are in the process of applying for benefits like Universal Credit – but who have not yet completed the process – do not qualify.
“Ultimately people are facing an impossible choice often,” said Citizens Advice spokesperson Katie Martin.
“They are taking a big hit on their incomes if they can’t work from home and they need to self-isolate, but they still have their bills to pay, their food shopping to do, children to care for.”
Ms Martin said the support on offer was welcome, but added: “It’s definitely something that we’d like the government to look again at – to see what else they can do to make sure people can do the right thing.”
Oliver, from Weymouth in Dorset, told the BBC that he discovered he was not entitled to any support after receiving a notification from the app to self-isolate.
“I have a young family, there’s seven of us who rely on my income,” he said. “I want to follow the guidelines, but they leave you in the lurch a bit without the financial support. It’s been a rough year – that financial burden feels like a massive injustice.”
The charity Working Families has also urged Chancellor Rishi Sunak to discuss eligibility, raising concerns that some working parents have had no choice but to miss work to look after children.
Many parents have children who have been forced to isolate because of cases in their school “bubble”, for example.
Shadow chancellor Anneliese Dodds said: “The prime minister is lucky he can self-isolate from the comfort of his home and carry on working, but for many people that’s not an option.
“Many people will be astonished to find that people using the Covid-19 app can’t access support to self-isolate – even if they’re eligible for the payment.”
She said the prime minister should “use his own period of self-isolation to fix this broken system”.
A spokesperson for the Department of Health and Social Care said self-isolation is “vital to stopping the spread of the virus” and added that a range of support is available to people on low incomes.
NHS Test and Trace contact tracers will make people aware of the support and tell them how to apply, the department said.
“We are actively exploring ways to expand the payment scheme to include those who are advised by the app to self-isolate because of close contact with somebody who has tested positive,” the spokesperson said.