Two young British tourists who met while backpacking have been murdered after partying on a beach at a popular island resort in southern Thailand.

David Miller, 24, from Jersey, and Hannah Witheridge, 23, from Great Yarmouth, were found half-naked with several deep wounds to their bodies near a beachside bungalow on the island of Koh Tao, Thai police said.

A nationwide manhunt was reportedly under way for a third British backpacker whom police suspect of carrying out the attack before fleeing the island.

A local police official told reporters that the pair had been murdered with a garden hoe found nearby on the island, which is popular for its serene beaches and diving sites.

Locals were reported to have blockaded Koh Tao's only pier to prevent the killer escaping. According to the Daily Telegraph, however, the local police chief, Colonel Songsak, said a British man who may have known Miller had left the island.

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Police Colonel Prachum Ruangthong said the male victim had sustained extensive injuries to his head and the woman had injuries to her face.

Investigators were checking CCTV from local bars and restaurants, and appealing for witnesses.

Ruangthong said a bloodied garden hoe used in the "gruesome" attack and bloodstained clothes were found near the bodies.

Another local police chief said the pair were believed to have travelled separately to Koh Tao, a popular island for backpackers in Surat Thani province, and met while staying at the same budget hotel beside the beach.

CCTV showed they went to the same bar and left together at 1am on Monday, said Major General Kiattipong Khawsamang. Local media reports said there had been a beach party for about 50 people, mainly tourists, on Sunday night that continued into Monday morning.

Witheridge is believed to have been travelling with three other friends while Miller is thought to have arrived on the holiday resort island with two others.

Local police official Jakkrapan Kaewkhao said the two travellers had arrived in Thailand separately on 25 August. "Their bodies were found 30 metres from the bungalow," he said.

An employee at the seaside resort where the pair had been staying told AFP that the bodies were found behind large rocks on the beach.

"It was the first time this has happened on the island, I have never seen anything like this," the staff member added, requesting anonymity.

The Foreign Office said: "We are aware of the death of two British nationals on 15 September on the island of Koh Tao in the Gulf of Thailand. The embassy is urgently seeking information from the local authorities and stands ready to provide consular assistance."

The mountainous island of Koh Tao, which is described on tourism websites as "an island of adventure action sports" and a place of "peace and tranquility", is popular with tourists but draws fewer travellers than the neighbouring island of Koh Phangan with its well-known full moon parties.

The double murder came the day before the half moon party, a similar event that attracts tens of thousands of people each month.

Thailand's lucrative tourism industry has been battered in recent months after a prolonged political crisis ended in a coup that saw the army suspend the constitution and impose a curfew and strict martial law on the country.

Although the curfew was swiftly lifted from tourist hotspots, visitor numbers have not recovered.

Thailand's military leaders have promised to clean up the kingdom's tourist resorts after complaints of scams, assaults and police extortion.