Dog walkers are being warned to keep a close eye on their pets after reports of three fatal poisonings in a park.
Police in Brent are warning people who regularly use Fryent Country Park following incidents of apparent intentional poisoning of animals.
Three dogs died from organophosphate pesticide poisoning last week within hours of ingesting the substance.
Owners who believe their dogs may have eaten anything suspicious are urged to seek help from a vet.
Officers are calling people to be on the lookout for any unknown substances near the roadside or in undergrowth, particularly any foodstuffs which might contain blue colouring, or any tablets or pellets.
The poisoning of animals is a criminal offence and anyone who may have information can contact police on 101 or CrimeStoppers on 0800 555 111.