Suppliers will be required to offer emergency credit to customers who cannot top up prepayment meters.
And if customers are in debt, suppliers must put them on "realistic and sustainable" repayment plans.
In March, suppliers voluntarily agreed with the government to support people affected by the pandemic.
Now Ofgem has updated its licence rules to formally require suppliers to help customers in financial difficulty.
The industry watchdog said those in financial distress would get some breathing space, but ultimately all customers will need to pay for the energy they use.
This follows Ofgem cutting the price cap on default tariffs and prepayment meters, due to falling gas wholesale prices, which means cheaper energy bills for millions of people this winter.
"Suppliers have stepped up to the challenge of supporting their customers during the Covid-19 crisis, especially those in vulnerable situations," said Ofgem's director of retail Philippa Pickford.
"Customers who are struggling to pay their bills should contact their supplier as soon as possible. The extra protections we have announced today will help ensure they get some breathing space this winter."
From 15 December, suppliers will be required to offer emergency credit or extra prepayment credit to households in vulnerable circumstances.
This could be because people are temporarily unable to afford to top up their prepayment meters, or are unable to visit their local shop due to having to self-isolate or having a mobility issue.
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