More than 400 schools in England have received a hoax email threatening to detonate a bomb on school grounds if money is not handed over, police said.
A number of schools were evacuated after receiving the emails, which demanded cash from them.
Humberside Police said counter-terrorism colleagues had advised that the threats were not “credible”.
Schools in London, Manchester and North Yorkshire were among those that received the email.
Police forces in Cumbria, Cambridgeshire, Humberside, Hertfordshire, Lincolnshire, Derbyshire, Avon and Somerset and Northumbria Police also issued statements saying schools and colleges in their areas had been targeted.
In Humberside, 19 schools reported receiving the threat.
Humberside’s Det Supt Tony Cockerill said: “We have spoken to all schools who have contacted us, reassured them that there is no need to evacuate and offered them security advice.”
Greater Manchester Police’s Assistant Chief Constable Vanessa Jardine said she understood parents and the general public would be very concerned, adding: “I want to assure you that we are working closely with all of the schools to ensure the safety of pupils and staff.
“We are carrying out extensive enquiries to understand the full circumstances and although there is not currently believed to be any direct threat, as with any report of this nature, they are all being thoroughly investigated.”
A North Yorkshire Police statement described the threat as a “hoax”, adding: “Our cybercrime unit detectives, supported by local officers, have looked at these incidents and it is not believed there is any genuine threat.”
Both the Home Office and the Department for Education, meanwhile, are advising any school that receives the email to contact their local police force – even though a version of the email that has been posted on Twitter warns the schools not to.
Some schools and colleges that were evacuated later said classes were continuing as normal.
Dowdales secondary school in Cumbria tweeted: “As you may know the school office received a threatening email this morning.
“The decision was taken to evacuate the school as a precaution. Students and staff are safe and have returned to normal lessons.”
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