Petrol prices have crept quietly to a record high, with hundreds of petrol stations charging more than £4 a gallon (87.9p per litre).

Hiding behind the fact that most motorists do not notice fractions of pennies added to the cost of a litre and can’t readily convert litres to gallons (the conversion rate is 1:4.55), the big oil companies have pushed pump prices through a psychologically significant barrier. BP garages now charge £4.01 per gallon on average. Texaco is charging £3.99, with price rises expected soon.

A survey carried out last week for The Sunday Times revealed that a quarter of the 6,000 stations studied were charging more than £4 for a gallon of unleaded petrol. There are about 10,500 fuel stations in Britain. Catalist, an independent supplier of information on the retail petrol industry, said it was the first time that a significant number of retailers were charging £4 or more.

The average UK price per gallon remains £3.97, but the AA has warned motorists to expect it to pass the £4 mark within weeks. The warning came as crude oil prices hit a high of $60.65 a barrel last week — 60% higher than this time last year and the steepest increase in more than 25 years, sparked partly by demand from developing countries, particularly China.

“While Opec (the Organisation of Petroleum Exporting Countries) has increased production there is a limit to refining capacity and this has almost been reached,” said Ruth Bridger, a petrol industry specialist for the AA Motoring Trust.

“The industry is struggling to come to terms with a massive increase in demand from China where the economy is growing at a phenomenal rate. All it takes is for someone to predict bad weather and the market will get nervous and the price of crude oil could shoot up.”

Since January average petrol prices have risen by more than 32p per gallon (7p per litre), costing motorists across Britain an estimated extra £5.1m daily in fuel.

Gordon Brown, the chancellor, agreed last week to freeze fuel duty for the second year running, in response to volatile oil prices.

The average motorist now spends about £1,150 per year on fuel, based on current prices and an annual mileage of 8,700 miles (average mileage, according to Department for Transport figures). Petrol duty accounts for about 47.1p per litre, or £2.14 per gallon, of unleaded fuel.

Petrol prices have not yet hit the highs, in real terms, that provoked protests in September 2000 when tens of thousands of motorists took to the streets. Then petrol leapt to £3.88 per gallon (85.3p per litre), the equivalent of about £4.36 (or 96p per litre) in today’s prices.

But prices are now higher in real terms than in the oil crisis in the late 1970s, when the price of crude shot up during the Iranian revolution. In 1979 a gallon of four-star petrol cost about £1 (22p per litre) — the equivalent of £3.40 (74.8p per litre) in today’s money, adjusted for inflation.

British motorists pay significantly more per litre than elsewhere in Europe. While we’re charged an average of about 87p per litre (£3.97 per gallon), the Swiss pay just 57p (£2.60 per gallon). Only Belgium, Holland and Norway have higher petrol prices than the UK, according to an American Express foreign exchange survey last month. Diesel crossed the £4 per gallon threshold in March and is now between 3p and 4p more expensive a litre than unleaded.

Many oil traders believe the price of oil could go as high as $75 a barrel before the end of the year — and oil companies are getting rich on the price rises. Goldman Sachs, the investment bank, estimates BP, Britain’s largest company with a market value of £123 billion, will pay out $13 billion (£7.4 billion) in dividends in the next two years and spend $24 billion (£13.5 billion) on buying back its shares to consolidate control over the company. Shell will pay dividends of $15 billion (£8.5 billion) and spend at least the same amount on acquiring shares.