A brazen thief stole items from an ambulance attending a 999 Christmas Eve call in Bryon Road, Wembley.
A paramedic and an emergency medical technician pulled up at 9.30am in Dickens Court to visit a patient. Fifteen minutes later they returned to their vehicle, which had been left unlocked and unattended, to discover an intruder in the back.
The suspect, who is described as Asian, 5ft 6in, approx 25 years old, wearing a black t-shirt and black woolly hat, fled on a silver mountain bike with medical books and personal effects worth a total of £100.
London Ambulance Service confirmed no controlled substances such as medicine was stolen.
Duty station officer Graham Card said: "Thankfully this kind of incident is rare, but it is totally unacceptable that when our crews are serving the public, their vehicle is targeted.
"We will do everything possible to aid the police in pursuing a prosecution."