A fire caused severe damage to Wigston Swimming Pool last night.
Firefighters were called to the blaze at the swimming pool, in Station Road, Wigston Magna, at about 9.30pm.
The fire started in the reception area of the building.
Four fire crews attended the scene and about 25 firefighters tackled the blaze.
A full search of the building was carried out and no-one was inside.
Severe fire and smoke damage was caused to the ground and first floor. Video by Liam Taylor
The swimming baths were closed earlier this year and are due to be demolished to make way for a new pool.
Firefighter Adam Taylor, from Wigston Fire Station, said the blaze started in the reception area.
"The heat and the smoke affected the whole building.
"It is suspected the fire had been burning for a couple of hours before the fire service was notified," he said.
The cause of the fire is believed to be deliberate and it was put out by about 11pm.
Watch manager Anthony Wildgoose, who also attended the blaze, said upon arrival the crew had to firefight from the outside before going in.
"There was lots of fire and smoke coming from the building and there were open doors on the premises."
He added that shortly after they arrived a window on the first floor gave way.
"We believe the fire had been burning for some time and the pressure had got so much it forced the window.
"We believe it is deliberate ignition," he said.
"Youths were seen fleeing the area a couple of hours before the call," he added.
The road was closed while the crews dealt with the incident.
Mr Wildgoose passed on the safety message that "disused buildings are not playgrounds."
Liam Taylor, 18, who is a student at Tresham College, in Corby said he and two friends were driving along Blaby Road, near to the pool on the way back from football training at about 9.45pm last night when they saw the fire.
"We drove past and there was lots of smoke. There were massive flames.
"It was coming out of the side window of the gym and was absolutely huge."
Liam, of South Wigston, added: "I could feel the heat from the other side of the road.
"After the fire was put out we could still feel the heat 40 to 50 metres (130 to 160 feet) away."
The building is owned by Oadby and Wigston Borough Council, and leased by Everyone Active.
Anne Court, the director of services at Oadby and Wigston Borough Council, said: "Our priority is working with the fire authorities so they can carry out the investigation to make it safe."
She added that the plans to demolish the building and build a new one that is due to open in May 2015, have not been affected.
Police also attended the incident and was called to the scene at 9.38pm.
A spokesman from Leicestershire Police said the fire is being treated as suspicious.
An investigation is being carried out and no-one has yet been arrested.
The police will be looking at CCTV in the area and is appealing for witnesses.
If you were in the area between 6pm and 9.38pm last night and saw any suspicious activity please contact the police on the non-emergency line 101 or Crimestoppers on 0800 555 111.
For more information about the incident follow @wigston_fire on twitter
