A man was left lying in the street for more than half an hour waiting for an ambulance to take him to hospital.
The 49-year-old collapsed in Station Road, Harrow on the afternoon of October 8, where a crowd gathered and an ambulance was called.
Fatjona Borizani, of Honeypot Close, was on her way home from college when she stumbled across the man and was the first person to call 999.
The 17-year-old said: "I called the ambulance at 3.34pm. That was the time my phone recorded it and I was waiting for ages to speak to someone."
She then saw the ambulance travelling in the wrong direction down Gayton Road and only after she notified passing police officers did a paramedic team arrive. When the fast response unit arrived, it then had to call another ambulance to take the man to the hospital.
Miss Borizani said: "When they got there the man was shaking.
"I thought it was ridiculous." According to the ambulance reports the man was not taken to hospital until 4.09pm, more than half an hour after the original call.
London Ambulance Service has a target to attend all emergency calls within eight minutes if the person is in a life-threatening condition, or within 19 minutes for non-life threatening conditions.
A London Ambulance Service spokesman said: "We were called at 3.34pm on Wednesday, 8 October to reports of a man collapsed on Station Road, Harrow.
"From the information received at the time of the telephone call it was established that the patient was not in a life-threatening condition.
"At 3.39pm further information from the caller showed the patient's condition had deteriorated.
"A paramedic and an emergency medical technician arrived at 3:49pm to begin assessing the patient, who was found to be fully conscious and breathing.
"After treating the patient at the scene, the paramedic requested an ambulance to transport the patient to hospital, which arrived four minutes later, at 4.09pm."