The poisoning of five dogs that appeared “stoned” on cannabis on Hampstead Heath remains unsolved after drug tests failed to provide any answers.

Blood samples taken from the pets were sent to a laboratory in the US after they fell into a stupor in the Sandy Heath area in April – but the results have come back negative for all toxic substances tested for, including recreational drugs.

Staff who treated the animals at the Village Vet practice, in Belsize Park, suspected cannabis or other drugs dumped on the Heath were to blame for the dogs’ conditions.

This now seems unlikely.

The animals had fallen ill after being let off the lead by a professional dog walker in Sandy Heath in April.

At the time Sean McInnes, head vet at the practice in Belsize Terrace, said the animals appeared “intoxicated”.

They were taken to the Village Vet and treated with emergency medication because their heart rates and blood pressure had plummeted to dangerous levels, with potentially fatal consequences.

They all made a full recovery.