The British government is investigating unsubstantiated allegations that British and Canadian soldiers are involved in smuggling heroin out of Afghanistan, a British newspaper says.

"We are aware of these allegations. Although they are unsubstantiated, we take any such reports very seriously," a defence ministry spokeswoman told the Sunday Times.

The Times said soldiers at the British base in Helmand province and the Canadian base at Kandahar Airfield are said to be involved.

A spokesperson at Canada's Department of National Defence would neither confirm nor deny that there is an investigation.

The spokesperson said that if there were an investigation, no one would say anything until it was complete.