Unlike consumers who use gas and electricity for heating and hot water, prices for households using heating oil are not capped by regulator Ofgem.
As a result, those who use heating oil have been among the first to feel the impact of rising crude prices and some customers say their costs have doubled.
The issue is particularly acute in Northern Ireland, where about 500,000 homes use it, almost two-thirds of all households.
About 3% of households in England and Wales said oil was their only source of central heating, according to a 2021 census, and 5% of households in Scotland.
Last week, Reeves said some heating oil companies were using the Middle East crisis as “an opportunity to rip off consumers” and asked the Competition and Markets Authority (CMA) to look into the issue.
In response, the UK and Ireland Fuel Distributors Association, which represents heating oil suppliers, said its members had seen “a very large and unexpected increase in demand”.
“We have spoken to many distributors who, despite the very large price swings and demand, are honouring orders as quickly as they can. We know that the CMA is monitoring this, and we support this approach.”
On Saturday, the head of the CMA, Sarah Cardel, said it was urgently looking into the issues and would “not hesitate to take enforcement action if potential breaches are identified”., external
Sir Keir is expected to raise concerns on Monday about reports of cancelled orders and prices being pushed up.
“If the companies have broken the law, there will be legal action,” he is expected to say.
