The latest package found in a Limerick postal depot had been addressed to Charing Cross in an apparent attempt to send it to the station.
Counter-terrorism police said on Friday evening that it appears to be connected to those sent to other major transport hubs in the capital, as well as to Glasgow University, earlier this month.
The package found on Friday was in similar white packaging and sent with the same stamps branded with a heart motif.
Deputy Assistant Commissioner Dean Haydon, senior national co-ordinator for counter terrorism policing said: “Whilst it is still early, from the images we’ve seen and from speaking to our counterparts in the Republic of Ireland, it appears that the package found in Limerick is linked to those sent to London and Glasgow on March 5 and 6.
“One line of inquiry is that the package found today in Limerick was returned there, having never reached the intended recipient in London.”
It was addressed to “Charing Cross, The Strand” with a postcode of WC2N 5HF, which is listed on the Network Rail website as being the station’s.
A group calling itself the IRA claimed responsibility for the earlier packages and said a fifth had also been sent.
Referring to the claim, Mr Haydon said: “Whilst that remains a line of inquiry, we continue to keep an open mind on who may be responsible and any potential motivation.
“I must also stress that we continue to urge the public to remain vigilant for any suspicious packages and to report anything suspicious to police.”
Irish Justice Minister Charlie Flanagan said “we have reason to believe” the latest package could be the fifth.
However, the group known to police as the New IRA had claimed on March 11 that the other parcel had been addressed to a British Army recruitment officer – not to Charing Cross.
Garda called army bomb disposal experts to the An Post depot in Dock Road on Friday morning after the suspicious package was discovered.
The Defence Forces said it was a “viable improvised explosive device”.
The first wave of packages arrived at Waterloo railway station and offices at Heathrow and London City airports on March 5 and 6.
They were posted with Irish stamps and had Dublin as the return address, prompting Irish police to join the investigation.
The latest also had the sender’s address as the republic’s capital, and added “Irelan (sic) tourism”.
All the stamps appeared to be those issued by the Irish postal service for Valentine’s Day 2018, featuring a heart motif and the words “Love Eire N”.
Police Scotland said a controlled explosion was carried out as a precaution on a suspicious package found in the mail room at Glasgow University, after several buildings had been evacuated.
All packages were A4-sized white postal bags containing yellow Jiffy bags and appeared capable of igniting a small fire when opened.
Speaking in Brussels, Irish premier Leo Varadkar said: “First of all I condemn the actions of whoever did this without reservation.
“No matter what is happening in politics at the moment, it’s no justification for violence and certainly no justification for potentially exposing civilians to injury or potentially death.”