Counter-terrorism officers were today continuing to investigate whether Irish terrorists were behind letter bombs that targeted the London transport network.

Sources said there were no signs a dissident Republican group was responsible for yesterday’s suspect packages, posted from Dublin to Waterloo station, London City Airport and Heathrow.

But they said the similarity to a New IRA campaign in 2014 meant the possibility was still being examined.

Letter bombs containing low-grade explosives and bearing a Dublin postmark were sent to a number of military careers offices in 2014. It was the first attempted dissident Republican attack on the mainland for a decade.

The latest explosive devices — sent in A4 postal bags containing yellow Jiffy bags — carried stamps saying “Love Ireland”. One burst into flames at Heathrow and two others were defused by bomb squad officers yesterday. Witnesses told of their shock at being caught in the attempted terror plot.

Father-of-two Kenneth Bradfield said he was “unnerved” by the aftermath at Waterloo. The 40-year-old data information specialist said he noticed something was wrong when he saw a queue of people waiting by the taxi rank.

He told the Standard: “The taxi area had been cordoned off, and there was a bus and blue car parked inside surrounded by police.

“I asked what was going on but all police said was that they found a suspect package. It was very well handled but still slightly unnerving.”

One man who works in City Aviation House, two minutes away from City Airport’s passenger terminal, said the building had been evacuated following a fire alarm.

The British Airways worker said: “The fire alarm went off and everyone made their way to the assembly point in the car park. We weren’t told what was going on, but I spotted on Twitter that there had been a security alert.

“We were told the building would not reopen and as I made my way home I could see police tape and police vans outside the building.” He added that he had “lived and worked in London long enough that a suspect package isn’t exactly an unusual occurrence”.

Police were alerted to the first package just before 10am after it ignited when opened at the Compass Centre, a building near Heathrow Terminals 1 and 2.

The second package was found at 11.40am in Waterloo station’s post room. Officers were told of a third package just after noon at Aviation House in east London.

Police confirmed no one was injured. There were no arrests.

Counter-terror police said they were “keeping an open mind” regarding any motive behind the packages.

Scotland Yard warned the packages appeared to have been capable of starting a small fire.

Sources said that although the New IRA is active in Londonderry, there have been no indications it is planning a mainland campaign.