UPDATE: Emergency services are still trying to determine the cause of a fire that destroyed the iconic Mill Street Tavern.
Fire crews are on the scene of the incident, where flames gutted the venue on Campbell St about 11.30 last night.
Darling Downs Acting Detective Inspector Paul McCusker said it would take time to reveal the cause.
He confirmed police were first on the scene after noticing the flames during a regular patrol.
"On arrival (at the scene last night), police observed that the premises were well alight," he said.
"Police attempted to enter premises to ensure no one was inside.
Queensland Fire and Emergency Services attended and extinguished the fire."
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EARLIER: A Toowoomba pub has been gutted by fire overnight.
The Mill Street Tavern at 142 Campbell St, Toowoomba was badly damaged by a fire which was reported about 11.30pm yesterday.
The pub is reportedly owned by former NRL player Jason Smith.
A spokeswoman for the Queensland Fire and Emergency Services said when crews arrived the fire was well involved.
"When crews arrived they called in a second alarm to get more appliances there," the spokeswoman said.
That meant four pumps were called out, bringing the total response to five pumps. A rescue appliance also assisted with efforts.
The entire top floor was well alight and crews were conducting search and rescue and fire suppression.
The north-east corner of the building collapsed.
Fire crews are watching the scene and there will be an investigation later today.
All persons were accounted for.
Police said they were investigating the cause a suspicious fire at a business in Toowoomba overnight.
"At around 11.30pm police patrolling the area along Campbell St observed large flames coming from a licenced premises," a representative said.
"The dwelling was unoccupied at the time and sustained significant internal damage."
A crime scene has been declared with further forensic examination of the site to be carried out this morning.
Anyone who may have seen anything suspicious in the area at the time of the fire is urged to contact Crime Stoppers on 1800 333 000.