Thailand has carried out its first execution in nine years in a move human rights campaigners condemned as “deplorable”.

Theerasak Longji, 26, was executed by lethal injection on Monday, six years after being convicted of aggravated murder for stabbing a 17-year-old 24 times in order to steal his mobile phone.

The brutal killing drew widespread anger in Thai society, and his conviction was upheld in the appeal and supreme courts.

​Theerasak’s execution comes as Thailand’s army chief-turned-premier Prayut Chan-ocha prepares to travel to the UK and France on a highly-anticipated official visit.

The last executions to take place in Thailand were of two drug traffickers. They were put to death in August 2009, after a period of no executions since 2003, according to Amnesty International.

Theerasak was the seventh convict to be executed by injection since Thailand switched methods in 2003 from a firing squad. Thailand has executed 326 people since 1930, according to the country's Department of Corrections.

The department said Monday that Theerasak was executed to be an example to those who think of committing serious crimes.

“Even though many countries have abolished the death sentence, there are still many other countries that still use it such as the US and China who focus on the protection of society and citizens to not become victims of crimes over the priority of the human rights of those who had violated the law,” the statement said.

Thailand is shockingly reneging on its own commitment to move towards abolition of the death penalty and the protection of the right to life Katherine Gerson, Amnesty's Thailand campaigner

“The Department of Corrections hopes that this execution will serve as a reminder to those who think of committing serious crimes or violating the law to stop and consider this sentence.”

Amnesty International said the execution was a major setback for the country.

“This is a deplorable violation of the right to life,” said Katherine Gerson, Amnesty International’s Thailand campaigner.

“Thailand is shockingly reneging on its own commitment to move towards abolition of the death penalty and the protection of the right to life, and is also putting itself out of step with the current global shift away from capital punishment.”

The Thai prime minister already faced pressure over his human rights record on his forthcoming trip to Europe.

Buddhist novices in Thailand: in pictures Show all 20 1 /20 Buddhist novices in Thailand: in pictures Buddhist novices in Thailand: in pictures Gorsak Kong Tawan (centre), 13, dances while he is carried on the shoulders of a relative Reuters Buddhist novices in Thailand: in pictures A woman smears thanaka bark paste, believed to have protective properties, on the head of one of the princes Reuters Buddhist novices in Thailand: in pictures Danusorn and Gorsak rest at an altar made for them at their home after the procession Reuters Buddhist novices in Thailand: in pictures August, eight, is joining the monkhood because he ‘wants to be a good person’ Reuters Buddhist novices in Thailand: in pictures Relatives dress August in imitation of Gautama Buddha, the founder of Buddhism, inside Wat Klang Thung temple Reuters Buddhist novices in Thailand: in pictures The first day of the ceremony begins with the boys having their heads shaved by their families Reuters Buddhist novices in Thailand: in pictures Boys with their heads smeared with thanaka bark wait to pray inside a temple Reuters Buddhist novices in Thailand: in pictures The boys are believed to have semi-divine status and are not allowed to touch the floor or walk for themselves Reuters Buddhist novices in Thailand: in pictures Phuwadol Kong Tawan, 11, Gorsak and Danusorn pray for a merchant at a local market during the procession Reuters Buddhist novices in Thailand: in pictures Poy Sang Long is Mae Hong Son’s most famous and colourful festival, held over the first weekend of April Reuters Buddhist novices in Thailand: in pictures The second day brings general merrymaking and a spectacular parade, with music and dancing Reuters Buddhist novices in Thailand: in pictures Each boy is accompanied by three attendants: one to carry him, one to shelter him and one to guard him Reuters Buddhist novices in Thailand: in pictures Gorsak is carried by a relative after using the toilet at their home in Mae Hong Son Reuters Buddhist novices in Thailand: in pictures After being ordained, the boys stay in the monastery for a period of weeks or months at a time during school holidays Reuters Buddhist novices in Thailand: in pictures They will learn the tenets of Buddhist teaching and the self-discipline required of a monk Reuters Buddhist novices in Thailand: in pictures The morning after entering the Buddhist novicehood the boys take part in an almsgiving walk through the town Reuters Buddhist novices in Thailand: in pictures Once accepted, the boys take vows, change into orange Buddhist robes and become full novices Reuters Buddhist novices in Thailand: in pictures The ritual morning almsgiving is a spiritual obligation that bonds local people with Buddhism Reuters Buddhist novices in Thailand: in pictures Boys play with their mobile phones and tables while staying at the Wat Klang Thung temple Reuters Buddhist novices in Thailand: in pictures Ten-year-old Danusorn Sdisaithaworn’s ordination will bring honour to his family Reuters

Human Rights Watch’s Asia director Brad Adams said in a statement released on Monday: “Prime minister Theresa May and President Emmanuel Macron should strongly express their deep concerns about the deteriorating state of human rights under military rule in Thailand.

“They should make clear to General Prayut that there will be no return to business as usual until Thailand holds free and fair elections, establishes a democratic civilian government, and improves respect for human rights.”

General Prayut has vowed to transform Thailand's government into a rights-respecting one and has scheduled general elections for 2015.