Four inmates were switched into a back-up prison van after a fifth man suffered a "medical episode" on a busy road just outside Liverpool city centre.
Police were present as the vehicle stopped on Vauxhall Road and paramedics attempted to help the prisoner who had fallen ill.
The incident led to dramatic scenes as the rest of the group being transported to Walton jail were moved into a second prison van.
Emergency services were called to the incident just before 5pm on Wednesday following concerns for a prisoner being taken from Liverpool's court complex to jail.
A spokeswoman for Merseyside Police said: "We were contacted at 4.50pm yesterday, Wednesday, May 12, following concerns for a male prisoner in a prison van on Vauxhall Road.
"The male, in his 30s, was checked by paramedics and taken to hospital by ambulance as a precaution following a medical emergency.
"The remaining prisoners were transferred to a second prison van and returned to custody."
The vehicle, operated by GEOAmey, had left Liverpool's Derby Square court complex minutes earlier.
A spokesperson said: “A GEOAmey secure escort vehicle was required to make an unscheduled stop in Vauxhall Road, Liverpool yesterday afternoon due to a medical emergency involving one of the prisoners on board.
"An ambulance attended to treat the prisoner and police also attended to assist with the transfer of the four other prisoners on board to a second GEOAmey escort vehicle.
"The ill prisoner was taken to hospital by ambulance, under escort, for treatment and later discharged to prison.”
