Over the past three days there have been suspicious packages discovered across London, Glasgow, Edinburgh and Essex.
Here is your guide to what happened where, and the latest on each of the packages.
Glasgow University, Dumfries Campus
Police arrive at the university's Dumfries campus. Image: Dumfries Police/Twitter
A suspicious package was discovered at Glasgow University's Dumfries Campus on Thursday afternoon [March 7].
The university branch was locked down at 12.30pm this afternoon by police and fire crews, at the exact time as as Edinburgh's Princes Street.
It is the second suspicious package to be received by the university, with a controlled explosion carried out at its main campus earlier this week.
Counter-terrorism police say the packages could be linked to three mail bombs delivered to London transport hubs earlier this week.
Princes Street, Edinburgh
Princes Street in Edinburgh. Image: Google Street View.
Edinburgh's busiest shopping street was evacuated on Thursday [March 7] afternoon due to a suspicious package found at a branch of Halifax.
Along with the bank, neighbouring shops the Carphone Warehouse, Three, Waterstone’s and Sports Direct were cleared of people, and Prince's Street has now been cordoned off.
A spokesman for Police Scotland said: “We received a report of a suspicious package having been received at a business premises on Princes Street at around 12.30pm today.
“The building has been evacuated as a precaution and officers are in attendance to progress inquiries."
Evacuation at the University of Essex. Image: Ed Brereton/Twitter
Students and staff at the University of Essex's Colchester campus were evacuated on Wednesday [March 6] after the discovery of a suspicious package.
A 100m cordon was placed around a section of the university after police were called at 11.50am.
The cordon was lifted at around 4pm today, after the Ministry of Defence sent an Explosive Ordinance Disposal team to the scene.
Essex Police have since said that the package "posed no risk to the public".
Lectures were cancelled following the discovery, but the university has said it will be open as usual on Thursday.
Police outside the University of Glasgow mailroom
A suspicious package was sent to the mailroom of the University of Glasgow on Wednesday [March 6], which was discovered just one hour before the University of Essex package.
Several university buildings were evacuated at 10.50am before bomb disposal officers carried out a controlled explosion of the package - which was never opened - as a precaution.
Police Scotland said they were working with the Metropolitan Police, following three explosive packages being sent to major transport hubs in London yesterday.
Sources have suggested the package could be linked to the three discovered in the capital on Tuesday.
London City Airport
Suspicious package received at London City Airport
At approximately 12.10pm on Tuesday [March 5], an improvised explosive device (IED) was discovered at London City Airport's City Aviation House.
It was the third to be discovered in London yesterday.
Staff were evacuated from the building as a precaution, and the package was not opened.
Specialist officers made the device safe, and the building was later reopened.
The suspect package, which was in an A4 sized white posal bag containing a yellow jiffy bag, was one of three sent out to Heathrow Airport and Waterloo Station.
The packages that arrived at the transport hubs were all posted with Irish stamps and had Dublin as the return address, prompting Irish police to join the investigation.
Specialists say they are capable of starting a “small fire when opened”.
The package sent to Waterloo Station
British Transport Police were alerted to a suspicious package in the post room at Waterloo at around 11.40am on Tuesday [March 5].
The package was the second explosive device to be discovered in London on Tuesday, and was not opened.
The station was not evacuated, but cordons were placed around a small area outside of the station where taxi's usually parked.
The partially-burned package sent to Heathrow Airport
The first of the three packages to be discovered was sent to Heathrow Airport's Compass Centre on Tuesday [March 5].
The device caught fire when staff opened the package, and police were alerted at 9.55am. Nobody was injured and no flights were disrupted.
Counter-terror police are still working to identify a motive or suspect over the explosive packages sent out to the three major transport hubs as sorting offices are on high alert for further devices.
Security sources suggested the packages' Irish insignia may have been a "concerted attempt" to make them appear as though they were posted from Ireland, but could not rule out that they had been.