West Yorkshire Police has launched an investigation - with officers "providing reassurance and safety advice to affected schools".

In messages to parents, one primary headteacher said police had been treating the threat as "credible" - and children were being kept indoors for the whole day, with gates locked and windows and doors closed.

In its statement, the force said: "Police are aware of an email which has been sent to a number of schools in West Yorkshire today.

Gildersome Primary School posted on Facebook on Thursday afternoon, saying staff are being "extra vigilant" and normal safeguarding procedures are being followed."

The school said it was "not in lockdown".

"To reinforce, there is no threat to our site, we are just being extra cautious as advised by the local authority."

"This is being investigated by police and officers are providing reassurance and safety advice to affected schools."

Earlier this week, a police investigation was launched after malicious emails were sent to a number of schools in the North West.

One parent told Sky News that she was "petrified" after she was told - and that it reminded her of incidents affecting schools in the US.

A school in Chester went into lockdown and prevented parents from picking up their children - but later said the "unpleasant email" it had received was "believed to be a malicious hoax".