Tens of thousands of Hong Kong-bound trucks and cars are filling up at a Shell petrol station near the border with mainland diesel that cannot be bought legally in the SAR because of air pollution standards. The station on the Guang-Shen expressway is the last before reaching the Hong Kong border. Under SAR law, filling a tank with mainland diesel, which has a significantly higher sulphur content and is half the price of Hong Kong diesel, is still legal. 'I go back and forth across the border every day, and I make sure to stop off at this station before I go to Hong Kong,' said a truck driver while filling his tank. 'Everybody does it - buses, trucks, you save yourself a lot of money.' Angela Spaxman, director of lobby group Clear the Air, said: 'It's not illegal and that's the problem. The incentive is there, but it's so unnecessary.' Hong Kong Customs law says that if a vehicle is bringing in more than 100 litres of mainland diesel, the driver must declare it and pay tax. Drivers say it is a common practice for everyone to fill up at the border and few declare it. Since they are rarely checked, drivers say they have no plans to stop unless the law changes. About 30,000 vehicles cross the border every day, and traffic is expected to increase 400 per cent in the next 10 years. Hong Kong has introduced proposals to shift vehicles to cleaner fuel, but cross-border co-operation on pollution in the Pearl River Delta is still hazy. Ms Spaxman suggested an immediate solution would be for Hong Kong to model itself on Singapore, where the Government limits the amount of fuel drivers can bring across borders to a fraction of the tank - and tests fuel levels before vehicles enter the city-state. Shell could not be reached for comment but green groups pinned the blame for the diesel problems on government policy-makers rather than corporations. Efforts to formulate policies are mired in bureaucracy in Hong Kong, they say. The Environmental Protection Department says it has had talks with its mainland counterparts on pollution, but a department spokeswoman said it could not comment on cross-border traffic issues that fell under Customs' watch. Customs said its job was enforcement, not environmental issues. Hong Kong's 110,000 trucks, light vehicles and buses account for 61 per cent of polluting nitrogen oxide and 67 per cent of respirable suspended particulates. Until laws change on cross-border regulation, drivers are likely to continue to opt for the cheaper, dirtier fuel. 'I have no problem following the law,' said one Hong Kong-bound tour bus driver. 'But so far, I am doing nothing wrong.'