Police are investigating claims a man operated as a 'fake' paramedic.
The 21-year old is believed to have treated more than 100 patients and responded to 999 calls.
Scotland Yard said it was contacted in July regarding concerns about the actions of a London Ambulance Service (LAS) member of staff.
The man was arrested in Hornchurch, Essex, in September, in relation to a period between May and July this year.
In a statement, police said: "The 21-year-old was arrested on suspicion of fraud, burglary, theft by employee, assault by beating, dangerous driving, driving otherwise in accordance of licence, and driving without insurance."
It is believed the man treated more than 100 patients while working at the LAS, according to the London Evening Standard.
He discharged patients at the scene and dealt with emergency calls, the newspaper reported.
The man was taken into custody at a south London police station and later released under investigation, Scotland Yard said.
LAS confirmed on Friday that it "immediately ended" the man's employment.
"We take our responsibilities to patients extremely seriously," LAS said in a statement.
"As soon as we became aware of the activities of this individual, we initiated a full internal investigation, informed the police and contacted patients affected.
"Following our investigation, we have improved, and will continue to improve, our security, systems and processes."
It added: "Changes designed to eliminate the possibility of anyone with internal knowledge of our computerised systems being able to misuse them in this way, have already been implemented."
A spokesman for the Care Quality Commission (CQC) said: "I can confirm that CQC is aware of this matter. As a result CQC carried out a two day unannounced focused inspection at the London Ambulance Service NHS Trust on 20 and 21 November 2018.
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"CQC will publish the findings of this inspection in due course."
According to the NHS website, prospective paramedics must be registered with the Health and Care Professions Council (HCPC) and have an approved qualification in paramedic science.