Starting from Tuesday, letters will be dropping on doormats across England and Wales after Sir Keir Starmer announced the expansion of the warm home discount scheme last year.
All households entitled to the discount are to receive a letter before January, with more than six million households across England, Scotland and Wales to receive help.
"This cash injection will help people manage their bills while we fix the rusting energy system we inherited," the prime minister said.
"Because it is only through our clean energy mission that we will get bills down for everyone in the long run, creating jobs and economic growth along the way."
Most of those eligible will have £150 automatically deducted from their energy bill without them needing to take any action.
Some though will need to provide extra information, such as an electricity bill or statement to confirm the bill payer's name and account number, to ensure they get the discount. Their letters will include advice for them to call a helpline.
"Letters will be dropping on doormats across the country this week containing welcome news for hundreds of thousands of families," energy secretary Ed Miliband said.
"This government is determined to tackle energy affordability for families, and this winter, more people will be helped as a result.
"And I would urge anyone who needs to provide extra information to follow the straightforward steps and make sure they get money off their bills this winter."
Ofgem's energy price cap rose by 2 per cent from 1 October for a typical household in England, Scotland and Wales, just as cooler temperatures see many switching on their central heating.
This means the energy bill for the average household paying by direct debit for gas and electricity increased from £1,720 to £1,755 per year.