The UK's largest supplier of heating oil has been forced to make sure its prices do not increase between when a customer places an order and the oil being delivered, after action from the Office of Fair Trading (OFT).

GB Oils has this month changed its domestic heating oil contracts to ensure prices quoted at order remain fixed until delivery, after the OFT secured legal undertakings from it (see our Cheap Heating Oil guide for tips on cutting costs).

Previously, when a consumer ordered from GB Oils, it quoted prices that reflected the market price of oil at the time. But when the customer came to pay on delivery, GB Oils hiked the price, saying the market price of oil had gone up in that time.

The OFT says in all the cases it looked at, the price on delivery was never lower than the quoted price.

If the company doesn't comply with the OFT's rules it can be taken to court, where it could face a fine or other form of punishment.

OFT senior director Mary Starks says: "Customers need to know where they stand when they are dealing with suppliers.

"The action we have taken will allow consumers to buy with confidence even during periods when the weather is snowy and prices are changing rapidly."

The OFT has also persuaded suppliers to change misleading information on their websites, and secured agreements from major liquefied petroleum gas (LPG) suppliers to improve transparency around switching and cancellation rights.