The bombs were sent in A4-sized postal packages to Heathrow, London City Airport and Waterloo station on Monday. One of the packages burst into flames when opened and others were made safe by bomb specialists. The packages also featured a circular heart with a "love from Ireland" stamp on them.
Security chiefs suspect the devices were a warning from Real IRA leftovers, who used a similar tactic of planting firebombs in London stores in the early 1990s. If the group is found to be responsible for the packages, it will be the first Republican explosives found on the mainland since 2014.
A source told The Mirror: "The New IRA are the prime suspects and their inner core is very tight. It is thought they are using the skills of older experienced people to create mayhem."
The stamps, according to The Sun, were bought in Irish post offices, but it is not clear whether they were stuck on by a sender or the post office. London and Irish police are working together on the investigation.
Sources told the Irish Times that the devices had been sent by post from Dublin but this hasn't been confirmed and no terrorist group has claimed responsibility yet.