Up to six schools across Leeds and Bradford have been closed with pupils locked in classrooms after a “credible threat” email.
The email is reported to have threatened violence against children and staff and is being treated as credible by police.
Parents have been told their children’s school has entered a soft invacuation, which means pupils will be kept inside all day, Leeds Live reports.
Gates, doors and windows have also been closed in response.
So far, schools in Hunslett Carr and Rothwell have received the warning, it is feared others could be impacted as well.
The list of schools so far is:
Richmond Hill Academy
Ruth Gorse Academy
Sharp Lane Primary
Windmill Primary
Beeston Primary
Clapate
Hunslett Carr Primary School in Leeds has not received the email, but has increased security to reassure parents and children.
Leeds Live reports West Yorkshire Police said: “Police are aware of an email which has been sent to a number of schools in West Yorkshire today.
“This is being investigated by police and officers are providing reassurance and safety advice to affected schools.”
Schools in both Manchester and Cheshire were also affected by a similar incident earlier this week.
Leeds City Council told Express.co.uk: “We are aware of an ongoing situation relating to threats made towards a number of schools in Leeds and we are working closely with West Yorkshire Police to monitor the situation and provide support to schools.”
They also said that schools have not been asked to lockdown but have been advised to be extra vigilant and continue to follow their robust safeguarding and site security policies.
Some schools may have initiated a lockdown as a precautionary measure whilst they await further advice from the police, they added.
Express.co.uk has approached West Yorkshire Police for comment.
