It is understood the prisoner, James Hogg, 38, of Dundee, was drunk on 'moonshine' - illicitly brewed alcohol. Inquiries have been launched by Humberside police, the Prison Service and Group 4.
Mr Hogg collapsed on Tuesday at 5pm as Group 4 took him from Rotherham magistrates' court to Wolds remand prison in Everthorpe, Humberside, also run by Group 4. He was under the company's supervision all day. According to some reports his condition went unnoticed for up to an hour as he lay in the van.
Last night he was in intensive care at Hull Royal Infirmary. He was joined there by a second inmate from the Wolds, suffering from a suspected overdose. The incident is thought to be unconnected.
Mr Hogg was appearing on charges with five others in connection with alleged illegal importation of cocaine. Reports from the police and court said Group 4 staff had failed to drop him at the Wolds despite handing over his commitment warrant. He remained unconscious in the van.
Paul Firth, clerk to the Justices at Rotherham, said he had been told the man's absence was not noticed until the van reached Hull prison at least half an hour later. The CID refused to comment on the allegations.
Mr Hogg and possibly one or more co-defendants apparently got drunk on moonshine hidden in a bottle which originally held spring water.
Derek Lewis, the prison service director-general, has called for a detailed report on the incident from his officials and Group 4 to be delivered within 36 hours. He has also appointed Alex Marnoch, a former commander in the Metropolitan Police, to head an investigation.
The escort service faced further difficulties in Yorkshire yesterday after prisoners taken to court by Group 4 were turned away for misbehaving. Only one of the six prisoners who had been driven from Moorlands young offenders' institute in Doncaster to Bradford magistrates was allowed in.
Group 4's problems began when a disturbance broke out in the van. Damage is thought to have been caused by the prisoners inside the vehicle.
'Because of the unruly and threatening behaviour of five of the prisoners at the magistrates' court, a police officer decided not to let them into court for safety reasons,' a police spokesman said.
Group 4 has lost eight prisoners since 5 April when it took over a contract worth pounds 9.5m a year to provide the first private prisoner escort service, based in the East Midlands and Humberside.Reuse content