“The NCA has taken the decision to conduct an investigation into potential criminal offences committed in the UK. This operation is underway,” said NCA deputy director Craig Turner.

The law enforcement agency, sometimes called the UK’s FBI, added that “at this early stage there are no confirmed links between the items purchased from the websites and cause of death in any of these cases”.

The announcement came after Canadian Kenneth Law, 57, was arrested in Ontario and charged with two counts of counselling and aiding suicide.

He is accused of selling a lethal substance to people across the world, with UK media reporting he sent 1,200 packages to 40 countries.

Police in the US, Italy, Australia and New Zealand are also investigating his actions.