WILLIAM TINNING and SIMON BAIN

Motorists are facing further misery after a warning that the continuing surge in crude oil prices to record levels could add another 20p to a litre of petrol and diesel at the pumps within weeks.

The forecast from the Institute of Advanced Motorists came as new figures showed car sales were tumbling in the face of rocketing fuel prices and a slump in consumer confidence.

The prediction could spell especially bad news for rural and remote parts of Scotland where people are more dependent on their cars.

In recent weeks diesel in some parts of the Highlands, which accounts for 25% of Scotland's A-class roads, was 14p a litre over than the national average.

Last month, the average price for diesel in the UK broke the £6 a gallon mark for the first time.

The dire warnings come amid fresh signs of a crisis in the High Street with John Lewis following on from Marks & Spencer in reporting a serious fall in sales.

Fuel prices rose to another record level at the pumps yesterday with unleaded petrol costing an average of 119p a litre and diesel costing an average of 132.4p a litre, according to the IAM Motoring Trust which monitors prices daily.

Yesterday the price of crude oil stood at $144.10 a barrel.

Prime Minister Gordon Brown signalled on Thursday that a planned 2p rise in petrol duty in the autumn will be dropped.

IAM Motoring Trust director Tim Shallcross yesterday said that the surge in oil prices in recent days had yet to feed through to the pumps.

He said: "If crude prices stay around their current level, we could well see another 20p on a litre over the coming month."

He added that petrol retailers had not passed on the full cost of the 100% rise in crude prices over the 12 months to the end of June. If they had, he said, unleaded would already be at 140p a litre on average and diesel 150p, with further rises to come.

Douglas Robertson, chief executive of the Scottish Motor Trade Association, which represents petrol retailers north of the border, said: "An increase of 20p a litre in the next few weeks is quite possible and would be really bad news for people who are dependent on their car for work and for other essential travel."

Alasdair Allan, SNP MSP for the Western Isles, who secured a Holyrood members' debate into fuel costs in May, said: "Already in the Western Isles we have the highest fuel prices in Europe. We can't take any more pain. We don't have a public transport alternative. We would benefit from a fuel price regulator of the kind that Labour MPs have rejected in Westsminster this week."

Warnings of higher fuel prices came as the Society of Motor Manufacturers and Traders (SMMT) said car sales totalled 209,000 last month - a 6.1% fall from May, and the steepest decline so far this year. Private (non-fleet) sales fell by 12%.

SMMT chief executive Paul Everitt said: "We are now seeing concerns about rising fuel bills and household costs dampening consumer confidence, leading to slower demand for new cars."

He said the rising price of fuel had encouraged people to change to smaller cars.

Meanwhile, the gloom across the High Street deepened yesterday when department store chain John Lewis said sales are running at 8% below their level a year ago.

It followed a warning two days earlier from the UK's premier retailer Marks & Spencer that its sales were down by almost 6%.

The three Scottish stores of John Lewis are seeing falls of around 4% in Aberdeen, 5% in Edinburgh, and 9% in Glasgow, although the group's Waitrose supermarkets across the UK are recording higher sales figures, largely thanks to the higher food prices on the shelves. Worst-hit were the big out-of-town John Lewis stores such as Bluewater in Kent, where sales are running at 25% below last year's level.

Freddie George, a retail analyst at City brokers Seymour Pierce, said the cost of petrol and economic worries meant the big malls were perhaps "being viewed as not attractive places to visit during the summer".

In the John Lewis stores, big ticket items such as washing machines and sofas have been among the main casualties of the consumer spending squeeze. In the week to June 27, electrical and home technology sales were the biggest fallers, down 16%, with home items 13% lower.

The rise in the cost of living over the past five years means that the average household is 15% worse off, according to an annual "discretionary income" study by accountants Ernst & Young, who predict that rising bills mean that "the worst could be yet to come".