Three bombs sent to London transport centres on Tuesday (local time) were reportedly sent by post from Ireland.
The devices, found in Heathrow and London City airports as well as Waterloo train station, "appear capable of igniting an initially small fire when opened," according to London's Metropolitan Police.
"The Met Police Counter Terrorism Command is treating the incidents as a linked series and is keeping an open mind regarding motives," they said.
British media, citing security sources, are now reporting that the parcels had Irish stamps on them, with Sky News journalist Alistair Bunkall posting an image on Twitter of what is believed to be one of the parcels.
One of the images shows a partially burnt parcel with red love heart stamps that are reportedly a limited 2018 Valentines' Day edition from an Irish post office.
A spokesperson for the Irish police says they are assisting the Metropolitan police with their enquiries.
The Independent reports that one of the packages was opened by office staff at Heathrow Airport and burst into flames, but no one was injured by any of the three devices.
London police are not commenting on the origin of the parcels.