Investigators trying to identify how fuel at scores of service stations came to be contaminated were focusing their attention yesterday on 30 huge petrol tanks on the Essex coast.

Thousands more drivers reported breaking down yesterday after filling up with contaminated petrol, and the number of complaints to Trading Standards soared.

The contaminated petrol is thought to have been stored and mixed at the Vopak terminal in Thurrock before being transferred to tankers for delivery by road. Laboratory technicians were working yesterday to identify the substance that caused the contamination. A quantity of silicon was named as the prime suspect.

The contaminant has caused oxygen sensors within vehicle exhaust systems to be knocked out, causing cars to lose power or come to a stop. The sensors assess the gases within the exhaust system and warn control units to raise or lower the petrol-to-oxygen ratio in the engine. When they burn out the cars either seize up or go into “limp home” mode.

Motorists furious at having to pay hundreds of pounds to get their cars repaired were further frustrated yesterday by a national shortage of spare parts. The unexpected run on sensors has swept garage shelves clear of them and stockists were forced to place extra orders with stockists abroard.

Vopak ordered its own investigation yesterday to find out which of its 30 tanks, which store three million tonnes of fuel each year, may have been contaminated.

The fuel, owned by the oil company Greenergy, which sold it to the service station owners, would have been tested coming into the site by sea to British standards before leaving by tanker, but nothing untoward was identified.

An abnormally high biofuel content of the petrol was originally cited as the most likely cause, but the probability of this receded when tests suggested that neither it nor diesel fuel was the cause. Traces of silicon were said yesterday to have been found in preliminary tests on batches of suspect unleaded petrol but it remained unclear whether this could have caused so many breakdowns.

Dr Richard Pike, chief executive of the Royal Society of Chemistry, said that silicon itself was unlikely because it is a solid, but silicon compounds such as liquid silicones, used in breast implants, have a number of uses in the oil industry including antifoaming systems.

He said that the industry would have to reassess its quality-control management and procedures and, if silicon in whatever form was found, routine testing regimes would need to be extended.

Trading Standards officials investigating the source of the contamination hope to announce preliminary test results at lunchtime today. The analysis of the petrol taken from broken-down cars should enable them to identify the likely contaminant and where in the supply chain it took place.

The number of motorists complaining to Trading Standards about the breakdowns soared from 200 by Wednesday evening to approaching 1,000 yesterday. David Broughton, of the Trading Standards Institute, said: “I have been doing this for 30 years and I’ve never come up against so many people complaining about exactly the same thing.” Mr Broughton added: “Everything is pointing to some form of contamination, perhaps even on the tanker ship that brought the fuel in. We don’t believe it’s sabotage.”

Motoring organisations were besieged by drivers reporting breakdowns or asking for advice, and the AA received more than double its usual calls.

The petrol blamed for thousands of vehicle breakdowns in the Midlands and the South East is thought to have been delivered to service stations on Wednesday last week.

Tesco, Morrisons and Asda were the worst affected, but other retailers caught up in the problem included Jet, Total and Texaco.

Trouble grows

145% increase in calls to the AA, measured in an hour before lunch yesterday

4,800 number of e-mails received by the BBC from motorists who fear their vehicles have been affected

1,000 approximate number of complaints about the fuel so far made to Trading Standards

£300 approximate garage bill to replace a failed oxygen sensor, although some were higher than £1,000

£5,000 penalty for supplying fuel that does not meet industry standards

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