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Another two flights were inspected in Warsaw, including one that was diverted after setting off from London Heathrow - another two landed in Sofia
A plane that departed Manchester was inspected in Murcia, where a local official said no explosives were found
We landed in Murcia - then the bomb squad turned up, one passenger tells us
On Monday, a flight from London Luton to Sofia made an emergency landing in Budapest, following a bomb threat
The Bulgarian transport minister has said there were bomb threats against two planes travelling to Sofia, local media reports.
Georgi Peev tells the state-owned Bulgarian News Agency (BTA) that there were threats against a flight travelling from the Italian city of Bergamo to Sofia - an incident we've already reported.
He also says a threat was made against a flight travelling from Dublin to the Bulgarian capital.
That brings the total number of flights reported threatened today to seven, in addition to another plane on Monday.
Peev explains that the flights into Sofia were not diverted and no explosives were found when the planes were checked.
A flight from Manchester to Murcia was searched by emergency services following a threat. Earlier a Spanish official explained no explosives were found on the plane and normal activity has since resumed at the airport. A dog was seen searching around the plane as firefighters waited nearby.
Avaition security expert Paul Mason says bomb threats on passenger planes "don't happen that frequently", which makes these incidents "unusual".
"It sounds like they've been made in a credible manner across a broad range of airlines," he tells BBC 5 Live. "That would suggest to me that it's been a very deliberate act by a typical actor rather than a load of coincidental, malicious calls."
Mason explains "there are a lot of processes" that the onboard crew and people on the ground must go through "to validate the credibility of the threat".
He says in such circumstances the primary responsibility for the crew is the safety of the aircraft and "maintaining a calm state in flight is absolutely essential".
Mason adds that the aircraft may be asked to divert if the credibility of a threat "is of a certain level" and in some cases the onboard crew may not even be advised on what the ground operators are dealing with.
Warsaw-Modlin Airport has reopened but officials are continuing to inspect an aircraft involved in a "reported bomb threat," the airport says.
"Passengers and crew remain in the arrival halls, awaiting further action," it says in its latest statement.
The airport adds that several areas remain closed until further notice.