A charity for single parents is warning thousands of children face poverty as parents stop maintenance payments.
Gingerbread has criticised a government decision allowing parents during the lockdown to change the amount they pay towards their child's upkeep without any investigation.
The Department for Work and Pensions said it did not have staff to check why some parents had altered payments.
But it warned parents who abused the system may end up in court.
The DWP said the coronavirus lockdown had led to an "unprecedented" rise in numbers of people claiming universal credit, which had required child maintenance support staff to be redeployed.
Gingerbread said it understood many separated parents not living with their children will have lost their jobs or getting paid less and would not be able to afford what they previously paid towards their care.
Charity chief executive Victoria Benson said: "The problem is that the single parents who relied on that payment in order to feed and clothe their children are no longer going to receive that money and quite frankly there's no other support that's been offered to the single parents."
She said calls to the charity's helpline had doubled.
The charity wants the government to make up any maintenance shortfall.
A mother, who asked to remain anonymous, from London said she had worked full-time but was furloughed as a result of the lockdown.
She has three children but is only receiving child benefit for one.
Her ex-husband stopped his weekly payments as soon as the lockdown was announced.
"I got a message from her father saying due to coronavirus he wouldn't be paying me any more," she said.
"I had a supermarket order - a slot I'd had for three weeks - and it was due to come that week just after the maintenance was paid.
"I had to cancel the food order, I was devastated, absolutely devastated.. it had left me short of food, left me short of money in the bank."
'Below poverty line'
She said her mental health had started to suffer and she felt herself panicking.
In a statement, charity Families Need Fathers said: "The government must urgently act on the recommendations of its own investigative body and ensure that under Universal Credit neither parent is pushed below the poverty line."
It added the government needed to ensure Universal Credit assessments took into account statutory child maintenance payments.
The DWP added: "No-one will get away with giving false information to avoid paying what they owe and all decisions carry rights of appeal, so either parent can dispute a decision."
According to Gingerbread there are about 1.8 million single parent households, nearly a quarter of families with dependent children.
About 90% of single parents are women, the charity said.