The blast happened at the Braeriach Hotel, Newtonmore, in the Scottish Highlands, early yesterday afternoon, and is thought to have destroyed the lounge bar, where several people were drinking.
Northern Constabulary said one casualty was airlifted by an ambulance helicopter to Raigmore Hospital, Inverness.
Another four people were taken in a fleet of ambulances by road to the same hospital. A police spokesman said that it was still too early to assess the extent of their injuries.
One woman who works in the Mains Hotel, close to the Braeriach, said: "It must have been a huge explosion because all the windows have been blown out and there is a real mess on the street.
"There were fire engines and ambulances everywhere. I think some people were badly hurt because the helicopter took them away. The traffic is being diverted away from the street."
Highlands and Islands Fire Brigade said that two units and a voluntary team from Newtonmore had attended the scene and had tackled a "minor" fire which broke out after the explosion.
It is thought that the blast may have been caused by a heating boiler, but police said it was too early to identify the source.
The police spokesman said: "The cause of the explosion is still not clear, but would appear to involve the boiler."
Newtonmore, off the A9 south of Inverness, is a popular town with New Year revellers. Local people said the Braeriach was one of the main hotels in the area.
Police said later there had been five people injured, not nine as previously believed. The man flown from the scene to Raigmore Hospital, Inverness was also described as stable and comfortable.Reuse content