A G4S driver allegedly stole £1m in cash from the van he was driving in south-west London.
The man is said to have stolen deposit boxes from the vehicle after parking it in Larkhall Rise, Clapham, on Tuesday.
Just over £1m was taken, the BBC has learned.
A 36-year-old man was arrested in Brixton on Wednesday on suspicion of theft after an investigation by the Metropolitan Police’s Flying Squad.
G4S, a major government contractor, said such incidents were “extremely rare” and there had been no “material financial impact” from the incident.
The Met said “a quantity” of cash was recovered.
Officers said there was no sign of forced entry to the vehicle and inquiries continue.
A G4S spokeswoman said: “We are working closely with the Metropolitan Police to resolve the matter.”