Fire investigators were examining claims the property was being squatted by homeless people.

Up to 50 firefighters tackled the fire at the former Central Hotel in St Mary Street which has resulted in major disruption for road and rail commuters in the city.

Six fire crews tackled the blaze

Three platforms at the city's Central railway station closed, bringing delays and cancellations to rail services between south Wales and London.

Much of St Mary Street was shut on Thursday as engineers look at how to shore up the walls of the fire-damaged building whose roof has collapsed.

No-one was injured the blaze which was reported at 0237 GMT on Thursday.

A security guard who was unaccounted for at the time was later found safe and well.

A spokesman for South Wales Fire Service said he understood people had been using the building prior to the blaze, although he still considered the fire to have started accidentally.

Mick Flanagan, acting assistant divisional officer, said crews had been forced to fight the four-storey blaze from the outside because it they could see the building was unsafe.

There are problems - it's a listed building, and obviously safety is paramount Incident co-ordinator Gwyn Jones

"Crews got to work straight away with hoses and started to enter the building, but the structure of the floor beams above the heads of the firefighters was bending, indicating that the floor was about to collapse.

"The inside structure of the building appears to be in the process of being refurbished or interfered with in some way so and this allowed the fire to travel very rapidly between floors and laterally across the building.

"We've liaised very closely with the police and Cardiff social services, who were able to tell us that some people used the building, squatting in there, some homeless people, and it's our belief that the most likely cause was an accidental start of this fire."

Rush-hour traffic

Rail services to and from Cardiff continue to be disrupted after the line was closed between Cardiff Central and Cardiff Queen Street stations.

The south Wales to London Paddington route has been badly hit, with train company First Great Western forced to delay or cancel a number of services due to the line side fire, reducing its half-hourly London service to an hourly service.

Fire service incident co-ordinator, Gwyn Jones, said structural engineers had been called in to shore-up the building, although this was unlikely to be done in time for Thursday's evening rush-hour traffic.

"There are problems - it's a listed building, and obviously safety is paramount," he said.

Six fire engines and three special appliances went to the scene.