An official anti-terrorist hotline for members of the public wanting to report suspicious activity is facing scrutiny over its effectiveness.
There have been reports that one of the London Bridge suspects was reported to the police through the phone number but that no further action was taken by the authorities. A similar claim was made about Salman Abedi after the Manchester bombing but police later said they could find no record of such a call being made.
People are urged to contact the police about possible terrorist activity including if someone has been buying large amounts of unusual chemicals or has a sudden need to rent a garage or shed.
Other signs may be suspicious use of cheques and credit cards or an individual holding documents in different names for no obvious reason.
Specially trained officers man the hotline round the clock, taking down the information, which is treated in the strictest confidence. Officers then assess the information that has been provided and a decision is taken on whether to send it to regional counterterrorism units to look at further and take action.
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In 2015-16 the hotline received about 22,000 calls but there are no published figures on what happened afterwards — such as how many were judged to be hoaxes or simple misinformation.
There are also no published figures on how many of the calls led to information being sent to regional counterterrorism units and acted upon.