The alarm was raised after Dutch police and customs officials seized sleepers awaiting shipment to Britain. The Dutch Environment Ministry said that the sleepers were treated with polycyclic aromatic hydrocarbons (PAHs) — a preservative that damages the environment when burned and can cause cancer.

A spokesman for the ministry said that the sleepers were only spotted because customs officials had stepped up checks at ports since foot-and-mouth disease spread to The Netherlands. He doubted that these were the first loads to be shipped to Britain.

It was unclear whether the sleepers had been ordered by the British Government or by private contractors.

The Dutch have alerted the environmental authorities in Britain and Belgium after receiving reports that the exporters might now attempt to ship the sleepers from Zeebrugge.

Over the past four days the Dutch police and customs have seized ten loads, each containing 300 sleepers, from the ports of Rotterdam, Scheveningen and Flushing.

It is illegal to export sleepers containing PAHs from The Netherlands. The public prosecutor will decide whether to press criminal charges against those responsible. The exporters have not been named.

The Ministry of Agriculture (MAFF) confirmed that imported railway sleepers from the Continent had been used in pyres. But a spokesman stressed that MAFF followed strict guidance on what could be used for pyres.