Police on Phuket are urging the public to remain calm while they investigate a large bomb discovered last month in an abandoned car outside the island's largest police station.

Phuket City Police found the explosive December 22nd when they were clearing a parking area in preparation for an annual inspection exercise to demonstrate security preparedness during the tourism high season.

Phuket Provincial Police Commander Ong-art Phiewruangnont ordered heightened security across the island.

"Of course we are concerned about this, but we don't want people to panic. At this stage we cannot confirm whether the bomb was operational or linked to the situation in the Deep South. The case is still under investigation. It is also possible that those responsible only meant to cause panic," he said.

Officers discovered an abandoned Isuzu pickup truck under a cloth car cover with license plates indicating it was registered in Nonthaburi province near Bangkok. It was stolen in May 2013 after its owner was murdered in Pattani.

Two gas cylinders containing more than 130kg of explosives were hooked up to a wristwatch inside set to detonate August 1st at 2.45pm– the same time a smaller IED in a Phuket Provincial Hall grounds trash can exploded, damaging cars, but causing no injuries. Pattani-based police Explosive Ordnance Disposal (EOD) unit has the evidence.

Bangkok-based ISH Jane's security analyst Anthony Davis said the Phuket bombs were the work of the Barisan Revolusi Nasional (BRN), according to Asia Times.

According to Davis, had the larger device exploded, "it would have demolished part of the police station and almost certainly resulted in a significant loss of life". Its likely impact on the tourist industry and economy "requires no additional commentary", he added.

Azfar Waeming, a Muslim tourist from Bangkok, told Khabar, "After hearing the news my confidence in Phuket as a safe destination was diminished somewhat. It is not like my previous trips here; I am more aware of potential dangers."

Phuket Tourist Association board member Jirachai Amornpairoj told Khabar the incident might have had a minor negative effect in the short term, but the trend remained positive.

"In December, more than 20,000 tourists arrived on the island every day, up about 1,400 persons per day compared to the same period in 2012," he said.

"The incident might affect tourist confidence to an extent," he said, "but the figures reveal that tourists still feel safe enough to travel here to enjoy the beauty of the island."