The 37-year-old officer from the port of Dover was detained by French police near Calais last Friday along with three other UK citizens, Britain's National Crime Agency (NCA) said in a statement.
The man was arrested in a supermarket car park in the rural district of Loon-Plage near Dunkerque.
Eleven firearms — nine handguns and two revolvers — were recovered and French police also seized 34 kilogrammes (75 pounds) of cocaine and seven kilos of heroin.
“The British officer, assigned to guard the British border, used his position to pass drugs and arms into Britain in his work vehicle, using the tunnel that passes under the channel,” a source close to the investigation was quoted in Le Point magazine.
“According to initial investigations, these drugs and arms were bought in Belgium and the trafficking has been going on for several months,” the source added.
Eight more people were then arrested in Kent in southeast England and in London, six of whom have been charged with conspiracy to import firearms and drugs.
The remaining two were released on bail pending further investigation.
“This operation has brought together law enforcement from both sides of the Channel, and we believe we have prevented the importation of a significant quantity of class A drugs and firearms to the UK,” said Dave Hucker, head of the NCA's anti-corruption unit.