The Competition and Markets Authority (CMA) said it has “heard a number of concerning reports from consumers who rely on heating oil about their experiences with retailers in recent days”.

An initial review of complaints has identified two main issues: existing orders for heating oil being cancelled, and customers then being offered new quotes at significantly increased prices; and price increases for automated deliveries (triggered when the fuel in a tank drops to a certain level).

The CMA said is writing to a number of firms – both direct suppliers and intermediaries – to seek further information about their practices.

Sarah Cardell, chief executive of the CMA, said: “We know many households are worried about rising prices as global events push up wholesale oil costs. It’s inevitable that some prices will rise, but they should reflect genuine cost pressures.

“We’ve heard troubling reports from heating oil customers about cancelled orders and sudden price increases – and are moving quickly to get to the bottom of these concerns. As always, we stand by consumers and will make sure they are treated fairly.”

Heating oil powers around 1.5 million UK homes, predominantly in rural areas, and is the main source of fuel for heating in Northern Ireland.