The driver alleged French police "tear gassed" his vehicle near Calais in an attempt to flush the occupants out.

The Public and Commercial Services Union (PCS) said one of its members was treated in hospital after searching the vehicle at the Kent port on Sunday.

The French police have refused to comment on the alleged incident.

'Streaming eyes

The lorry was stopped and searched on arrival at the Port of Dover by a female customs officer.

She suffered nausea, streaming eyes and "felt disorientated" within minutes of entering the back of the vehicle.

Another officer was told by the driver that French police using teargas had found two stowaways in Calais, before allowing the lorry to cross the English Channel.

Another five people were found inside the lorry in Dover, all of whom had to be treated for nausea at the scene.

'Dangerous substances'

Pete Norris, of the PCS union, said the incident had caused him great concern.

"If there should be a repeat of this set of circumstances our members could regularly be coming into contact with very dangerous substances such as teargas when they look into the back of lorries for other goods."

The Port of Dover police confirmed that an incident had taken place, and was being investigated with the French authorities.