Managers have been told they must either quit their mainly single-forecourt operations and take on clusters of six, or leave the franchise. Shell claims the stations are selling more petrol but profits remain slim.

The oil group's move comes at a time when drivers are paying high prices for fuel but oil companies say this is due to increased crude prices and high fuel taxes rather than profiteering.

She had tried in the past to run two sites at the same time but found it tough, if only because of staffing problems. "Remember these sites are open between 16 and 24 hours a day, seven days a week. When a member of staff fails to turn up one morning you tend to do it yourself. Imagine what it would be like with six sites. It makes me sick even to think of it."

Ms Martin has spent 14 years working at a Shell station but in January will leave her post at the Castle View site near Denbigh in north Wales, as will Donna Rose at nearby Rhyl. "I am out of here in three weeks' time. I looked at the conditions that Shell required in terms of a £100,000 bank guarantee and a £100,000 personal guarantee and decided it was far too much," said Rose.

Rose believes that the reorganisation is just a cost-cutting exercise that is unworkable in rural areas, where petrol outlets might be 30 miles apart from each other. "They [Shell] have already cut the budget for shop and forecourt cleaning but trying to say that this was against customer needs is like banging your head against a brick wall."

Shell accepted that some of its franchisees on these company-owned sites would inevitably not want to take on the extra responsibility and commitment but the company denied that the move was about reducing costs.

Simon Grimsell, Shell's retail sales manager for Britain and Ireland, said: "I fully acknowledge this will not be for everyone but we are confident that this model can be successful because we have similar ones operating in New Zealand and Norway. It has led to improvements."

The company has been engaged in a long period of consultation with franchisees and had given them a long period of notice about the changes, which come with significant rewards. A commission is paid on every litre of fuel sold and shop takings are shared 50/50. There are also allowances for operating costs, including staff and supervisors.

There are now 370 owners running the 600 sites and while 250 of these wanted to go ahead with the new "cluster management" scheme, only 117 were selected.

Shell denied that the remainder would therefore automatically lose their jobs. It said some might be participating in some of the partnerships being formed to take over some of the six-site clusters. Others might be hired as supervisors.

The oil company declined to say what level of profit it was making from selling petrol but said competition from supermarkets and others made the market "highly competitive". It said fuel retailing was a growth sector with overall sales making double-digit growth annually for the last four or five years.