The police are stressing the incident is not being treated as terrorist related.
A police spokesperson said: "On 29 March acting on information received police stopped and spoke to a woman in a car park off Pudding Mill Lane. Her vehicle was searched and the 40 year-old woman was arrested on suspicion of possession of an explosive substance and class A drugs. She is in custody at an east London police station."
"The arrest was made by the officers from the Olympic Site Support Unit following information received. Another car was stopped and searched by the OSSU on the M11. Nothing was found in the car, and the driver was not arrested. Searches were carried out at residential addresses in Kent and London, no further substances were found.
"The investigation is being carried out by Newham CID. The Counter Terrorism Command is aware but the incident is not being treated as terrorist related."
A G4S spokesperson said: "Our canine services team is licensed to hold small samples of explosives for training purposes and are required to undertake rigorous training and follow strict operational processes.
"Our ODA canine services teams are highly effective because the dogs are able to regularly train using ‘live’ training aids. All dog handlers are fully vetted and operate to the highest industry standards."
"G4S take breach of operational processes very seriously and are assisting the police with their enquiries in relation to this incident."
G4S Canine Services is currently reviewing its policies and procedures.
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