Schools in Italy, Scotland, Northern Ireland and Ireland have received bomb threats.
It is the latest wave of hoax alerts to hit Europe.
The warnings caused schools in Italy to be evacuated, disrupting the day of parents and pupils.
Schools across Italy have been evacuated after around 6,300 of them received bomb warnings, the latest in a series of threats and cyber attacks around Europe against supermarkets, blood banks, air traffic and broadcasters.
Scotland, Northern Ireland and the Republic of Ireland also said many of their schools received a similar email threat from abroad, but it was classed as a hoax and did not disrupt classes.
In Italy, schools closed swiftly after the warnings, prompting many parents to drop what they were doing to collect children.
"My daughter Dafne started crying because the kids did not understand what was happening, and she was still shaken when I picked her up," said Nensi Bego, parent of a child at the Mazzini middle school in Rome.
"This is so annoying. It's obvious it's just a hoax that disrupts lessons and disrupts everyone's day."
A copy of the message seen by Reuters also included a threat to target Prime Minister Giorgia Meloni. Ms Meloni said the bomb alerts posed no danger and that police and other public bodies were monitoring the situation.
