Meanwhile, a major London underground station was shut down briefly because of a fire alert that proved to be a false alarm.
Three small explosive devices in plastic mailing bags arrived at offices for two London airports and at a train station Tuesday, and Irish police said they were helping British counterterrorism police with the investigation.
London's Metropolitan Police Service said the devices found near London's Heathrow and City airports and at Waterloo Station "appear capable of igniting an initially small fire when opened." A staff member did unseal the bag sent to an office building on Heathrow's grounds, "causing the device to initiate," the police department said.
Part of the package burned, but no one was injured, police said.
The Compass Centre was evacuated as a precaution. The building is not close to passenger terminals at Heathrow, Britain's busiest airport, and flights were not affected.
The Heathrow property is where the first of the three explosive devices was discovered at about 9:55 a.m.
Transportation police received a call at 11:40 a.m. after another suspicious package turned up in the mail room at busy Waterloo Station. The last was found just after noon at the administration building for City Airport, a much smaller commercial airport.
No one had opened either of those packages.
The Metropolitan Police Counter Terrorism Command "is treating the incidents as a linked series" and "keeping an open mind regarding motives," the department said in a statement.
Sky News correspondent Alistair Bunkall tweeted a picture of what he said was the package sent to the Heathrow office building. The stamps on the envelope have pictures of hearts and resemble ones issued by Ireland's post office for Valentine's Day 2018.
Police in Ireland told The Associated Press on Tuesday evening they were assisting British investigators, but did not give an explanation or details.
No person or group claimed responsibility for sending the devices and police said they haven't made any arrests.
Train services to City Airport were suspended temporarily and the administrative building, City Aviation House, was evacuated. Flights were not affected.
Police advised transportation stations throughout Britain to "be vigilant" and to report suspicious packages. London Mayor Sadiq Khan urged London residents and visitors to do the same.
The official terrorism threat level throughout Britain is set at "severe," indicating that intelligence analysts believe an attack is highly likely.
Major underground station briefly shut after false alarm
A major London underground station shut down briefly on Tuesday because of a fire alert that proved to be a false alarm.
Kings Cross St Pancras underground station, an interchange that serves two long-distance rail stations, closed for about half an hour after reports a fire had broken out.
"We believe that it was a hoax call at one of the fire points," a Transport for Londo n spokeswoman said, adding that investigations into the cause of the alert were continuing.
A spokesman for London's fire brigade said there had been a false alarm and that firefighters had established there was no fire within 10 minutes of arriving at 2144 GMT.
The underground station was the scene of a devastating fire in 1987 that killed 31 people.