Dissident republican group the IRA have claimed responsibility for letter bombs that were sent to London transport hubs and the University of Glasgow last week.
According to the claim, five packages were sent, but only four have been accounted for.
The Met said: “Extensive advice has already been issued to relevant businesses and sectors to be vigilant for and report suspicious packages to police. This advice was previously sent to armed forces personnel and is being reiterated again in light of this claim.”
The Met said it was already looking into Northern Ireland-related terrorism as a possible source of the packages, which had Irish stamps and a Dublin return address, however, the police said they were keeping an open mind.
“We continue to urge the public to remain vigilant and report anything suspicious to police,” the Met said.
The Irish Republican Army (IRA), sometimes called the new IRA in the media, was formed in 2012 when Republican Action Against Drugs and other small dissident republican groups merged with the Real IRA.
The Real IRA had split from the Provisional IRA in 1997 after the latter agreed to a ceasefire, with its political wing Sinn Fein admitted to peace talks and going on to support the 1998 Good Friday Agreement.