Emergency help for poorer households hit by energy bills driven up by the Iran war is to be unveiled by Sir Keir Starmer.
The government says it will provide "targeted" support for families that rely on heating oil, the cost of which has risen by 80% in a week.
The prime minister is also threatening legal action, which could include fines or other financial penalties, against suppliers who rip off customers.
"It's moments like this that tell you what a government is about," the prime minister will claim at a news conference in Downing Street.
"My answer is clear. Whatever challenges lie ahead, this government will always support working people.
"That is my first instinct - my first priority - to help you with the cost of living through this crisis."
The PM will also hit out at claims that some suppliers are cancelling orders for heating oil and then "jacking up prices".
He will say: "I will not tolerate companies trying to exploit this crisis to make money from working people. If the companies have broken the law, there will be legal action."
The crackdown follows the PM's visit to Northern Ireland last week, where around 60% of households use heating oil and have seen a sharp rise in prices since the start of the war.
In a Times interview on Saturday, Chancellor Rachel Reeves promised "targeted" support for poorer households and said a universal bailout for every household would be unaffordable.
"I've found the money and we've worked through with MPs and others a response for people who are not protected by the energy price cap," she said.
"We are giving greater support to those who really need it."
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On Sunday Morning with Trevor Phillips on Sky News, energy secretary Ed Miliband said: "We're going to fight people's corner in this conflict.
"That's why we've been saying to the petrol retailers, the heating oil companies, we're not going to tolerate price gouging. We're not going to tolerate unfair practices."
Downing Street sources claim the PM has acted in the national interest "at every turn" and is working with international allies towards a swift resolution to the war - the quickest way to stop it impacting working people.
"We will continue to work towards a swift resolution of the situation in the Middle East," the PM will say. "Because there's no question that ending the war is the quickest way to reduce the cost of living."
(c) Sky News 2026: Keir Starmer to unveil emergency support for households worst hit by energy price rises
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