Polish authorities said four people were arrested over “parcels containing camouflaged explosives” that were allegedly sent via courier companies to countries including the UK.

Poland’s National Prosecutor’s Office said the arrests related to the sending of parcels “which spontaneously ignited or detonated during land and air transport” to EU countries and the UK.

Prosecutor Katarzyna Calow-Jaszewska said the group’s goal was allegedly “to test the transfer channel for such parcels, which were ultimately to be sent to the United States of America and Canada”.

Four people involved in the activities of the revealed sabotage and diversion group of an international nature were detained, charges were announced, and then temporarily arrested

It has been reported that counter-terrorism police are investigating whether Russian spies planted a bomb in a parcel that caught fire on July 22 at the DHL warehouse in Minworth, near Birmingham.

The package – said to contain an incendiary device – is believed to have arrived at the DHL warehouse by air but further details about the plane and its flight path are unknown.

Counter-Terrorism Policing said on Monday evening the arrests reported by the Polish authorities were not carried out as part of its investigation.

Ms Calow-Jaszewska said: “During the proceedings, four people involved in the activities of the revealed sabotage and diversion group of an international nature were detained, charges were announced, and then temporarily arrested.”

In a statement on October 25, Ms Calow-Jaszewska added: “The group’s activities consisted of sabotage and diversion related to sending parcels containing camouflaged explosives and dangerous materials via courier companies to European Union countries and Great Britain, which spontaneously ignited or detonated during land and air transport.

“The group’s goal was also to test the transfer channel for such parcels, which were ultimately to be sent to the United States of America and Canada.”