A stash of heroin worth £16 million has been seized after police were alerted by a lorry driver when he became suspicious of the load he was carrying.

Police were called by the driver at around 1pm on Thursday. Following a pick up in East London some time earlier, he had stopped in Letchworth, Hertfordshire.

When officers searched the vehicle, 40 kilos of what is believed to be heroin with an estimated street value of £16 million was found.

Hertfordshire Police said officers are working with the National Crime Agency to “relentlessly pursue and arrest” those responsible for transporting the drugs.

“A significant quantity of drugs has been removed from criminal circulation and I have a team of officers investigating who is responsible for their importation and intended distribution,” detective superintendent Peter Fulham said.

“Hertfordshire Constabulary, working with the National Crime Agency, will relentlessly pursue and arrest those involved in the supply of a class A drug that causes such harm. Whilst our proactive work is on-going, information from members of the public could prove vital.

“I am appealing to anyone who has information regarding the importation of these drugs or knowledge of individuals who may have been involved in the supply to contact the Hertfordshire Constabulary with information. If you do not want to talk directly to police, please call Crimestoppers anonymously,” he added.

A spokeswoman for Hertfordshire Police told the BBC it would not comment on the driver’s reason for becoming suspicious for “operational reasons”.