A strike by petrol tanker drivers started to bite yesterday as motorists were urged to stay calm and only drive if absolutely necessary. With the Army on standby and the prospect of a second action next week if talks fail again, hauliers supplying Shell stations walked out at 6am.

Strikers picketed terminals across the UK and some drivers working for other companies refused to cross union lines, further squeezing supplies. Shop stewards said support was rock solid among their members. But despite reports of queues forming and some stations running dry, retailers reported demand only 10 per cent up on a normal Friday.

The four-day stoppage threatens seriously to test the resilience not only of the UK's fuel supply chain but the ability of motorists – already buckling under the pressure of soaring prices at the pump – to heed official advice to carry on as usual. Effects that will become more pronounced over the weekend could be felt more keenly in some parts of the country than others. Most Shell stations are in central Scotland, the North-west, the Midlands and the South-east.

However, union leaders in Devon and Cornwall said the situation could deteriorate rapidly with supplies drying up from last night.

A Unite official said the two Plymouth plants were the only ones supplying fuel for the two counties and both were being picketed. The next-closest depot was Avonmouth in Bristol and 140 drivers representing every fuel company were supporting the action.

All 150 drivers on Shell contracts walked out at Shell's Stanlow refinery at Ellesmere Port, while at Grangemouth, Scotland's main fuel depot, only a handful of tankers had left by yesterday lunchtime. The dispute is due to continue until Tuesday. Gordon Brown urged both sides to resume negotiations. He said that contingency plans were in place.

A Unite spokesman played down the significance of the fact it had already notified the management of the two Shell sub-contractors, Hoyer and Suckling Transport, of a possible second strike next week describing it as a "legal formality".

"We are focusing on getting a resolution over the next few days," he added.

However, one negotiator, Gordon Johnson at Grangemouth, said: "After today, when they see what's been going on people will start to take us seriously. We've already started to see people queuing up for petrol."

The Petrol Retailers Association said demand appeared to have spiked on Thursday when sales were up by 20 per cent. A spokesman said the reduced demand showed that drivers were heeding repeated warnings from Government not to panic buy.

What do they want?

*The drivers' union, Unite, says its members' pay has not risen since 1992 and is demanding a 13 per cent increase. The haulage company Hoyer and Suckling Transport says it has offered a 7.3 per cent rise backdated to 1 January, which would take a driver's average earnings to £39,000, and a further 6 per cent increase from 1 January 2009, taking earnings to £41,500. The company says the union refused. Tony Woodley, Unite's joint leader, disputes the figures and says his members' basic wage is £31,800, which he wants to see rise to £36,000. Had Shell not outsourced the drivers' jobs, they would have been on £46,000, he claims. Drivers say changes to pensions have caused many to delay retirement. They also claim changes in working practices, such as being solely responsible for transferring petrol from tankers to filling stations, have made their jobs more dangerous.