At 0522 GMT, a coach crashed into a tree after skidding on ice on the A40 at Puesdown, south east of Cheltenham.
It was carrying 29 Royal Marines from 42 Commando in Plymouth to RAF Brize Norton in Oxfordshire, from where they were due to fly to Afghanistan.
The 39-year-old civilian driver, who was cut free after nearly two hours, is thought to have serious head injuries.
A Ministry of Defence spokesman said six marines were also hurt. Five are due to be discharged later and the sixth is currently being examined in hospital. The families of those involved have been informed.
A Gloucestershire County Council spokeswoman confirmed that the road had been gritted twice overnight.
Earlier, at about 0445 GMT, a minibus with 14 people on board crashed at Highnam, near the A40 west of Gloucester.
Twelve people were hurt, one seriously, and all were taken to Gloucestershire Royal Hospital.
Serious incidents
An ambulance spokesman said a number of paramedic crews had been sent from across the Gloucestershire and Avon region.
"Clearly these are both very serious incidents with multiple patients injured," he said.
Gloucestershire Police received reports of more than 60 accidents between 2200 GMT on Tuesday and 0900 on Wednesday due to icy conditions.
Further north, a West Yorkshire police spokesman said black ice had caused difficulties and minor accidents on many roads, including motorways.
A caller to BBC News said vehicles were parked on the hard shoulder of the M621 after sliding into the central reservation barrier.
In Wiltshire, police say they had received 40 reports of accidents since 0600 GMT, while Warwickshire saw 24 crashes before 0900 GMT.
The county's highways manager said roads were gritted on Tuesday, but flood waters washed much of the salt away.
In Bedfordshire, police closed a number of roads and the council cancelled several school bus services because of icy conditions.
A Derbyshire school was shut after an 11-year-old boy suffered head injuries falling on ice as he walked between lessons at Mill Hill secondary school in Ripley.
In North Yorkshire, a woman hurt in a crash on an icy road near Harrogate, North Yorkshire, has died.
BBC weather forecaster Rob McElwee said the sun was out across much of the UK but another freezing night would bring widespread black ice on Thursday morning.
Motoring organisation, the AA, said black ice - a thin, nearly invisible coating of ice on paved surfaces - was "particularly dangerous".
Head of public affairs Paul Watters told BBC News gritting was not "enormously" helpful and advised motorists to drive more slowly and keep their distance from the car in front.
Rivers overflow
Elsewhere on Wednesday a clean-up operation was under way following Tuesday's floods.
There are about 150 flood alerts of varying degrees still in place and water is still standing in some areas, including around the flooded River Severn in Gloucestershire.
Paddy enjoys the floods around the River Severn in Gloucestershire
Several rivers burst their banks including the River Colne in Hertfordshire, where some areas were under four feet of water, and the Cam in West Camel, Somerset.
The Severn Bore - a tidal wave which flows along the narrow Severn Estuary - reached "three star" level earlier on Wednesday and flooded surrounding countryside, according to the Environment Agency.
Local authorities warned spectators to take care on Thursday, when a more powerful four-star bore is predicted.
London's Thames barrier was closed for a second day to protect the capital from rising waters.
A spokesman for the Environment Agency said: "We are not expecting any escalation to severe flood warnings.
"It has stopped raining and the water is moving naturally through the system. River levels are dropping across the country."
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