Thai opponents of the ousted Prime Minister Thaksin Shinawatra threatened a mass rally against him as he arrived in Cambodia today, on a visit that has brought tension between the two neighbours to fever pitch.

Mr Thaksin was welcomed by his host, the Cambodian Prime Minister, Hun Sen, whose appointment of him as an “economic adviser” has infuriated the Thai Government. The anti-Thaksin activists known as the “Yellow Shirts” announced a mass rally against the former Prime Minister in Bangkok this Sunday, after remarks which he made in an interview with The Times about the country’s royal family.

Previous Yellow Shirt rallies have provoked fights with Mr Thaksin’s supporters and brought chaos and paralysis to Bangkok. Last year the group took over Government House and seized both the city’s airports, stranding tens of thousands of passengers and damaging the country’s tourism industry.

Now, tensions are at a peak again as Mr Thaksin returns to southeast Asia, the closest he has been to Thailand since he was forced out in a military coup in 2006.

On Cambodian television, Mr Hun Sen was seen embracing his guest and was said to have described him as an “eternal friend” and “the best adviser with economic leadership”. On Thursday Mr Thaksin will give a lecture to an audience of ministers and government officials.

“Thaksin is here for the economy and no activities related to politics,” Phay Siphan, spokesman for the Cambodian Cabinet, said. “It is an honour for Cambodia's economic sector and we hope that Cambodians nationwide welcome him warmly.” But it is clear that Mr Hun Sen’s hospitality has less to do with Mr Thaksin’s economic expertise than with the fraught relationship between Phnom Penh and Bangkok.

Historically, their differences date back to the medieval Khmer Empire, a Cambodian civilisation that ruled over large parts of Thailand. Nowadays, Cambodia is much poorer than its former vassal, but a sense of resentment and rivalry lingers on both sides that periodically flares into violence and vitriol.

Last year, a handful of soldiers on both sides died in small skirmishes over a few hundred square metres of disputed territory close to the ancient Preah Vihear Buddhist temple. Thailand’s current Foreign Minister, Kasit Piromya, is a Yellow Shirt leader who, before his appointment described Mr Hun Sen as a “a gentleman with the mind of a gangster”. Now the Cambodian leader is taking his revenge by flaunting his friendship with the Thai Government’s bête noire.

Thailand has responded by recalling its ambassador to Phnom Penh and moving to cancel an agreement jointly to explore energy reserves in the Gulf of Thailand. It has begun extradition proceedings against Mr Thaksin, who last year was sentenced to two years in prison for corruption perpetrated when he was Prime Minister. It has no hope of success, since Cambodia regards the case as a political matter, and therefore excluded from the extradition treaty between the two countries.

Mr Thaksin told The Times at the weekend that he has no intention of settling in Thailand but will visit from time to time from his exile in Dubai. But his proximity to Thailand, and especially to the country’s northeast where he has his most loyal support, is deeply unsettling to the Government, which is supported by army generals who propelled Mr Thaksin from power.

In April Mr Thaksin promised his supporters: “If there is the sound of gunfire, of soldiers shooting the people, I'll return immediately to lead you to march on Bangkok.”. On Saturday he told The Times: “If I were to start the march I would start from the northeastern part of Thailand, on the soil of Thailand, but I will have to enter Thailand from the border. I can enter Thailand from Laos, Cambodia, Myanmar.”

“Tonight I will meet and have dinner with Hen Sen and his family,” he wrote today in a message to his supporters on his Twitter online feed. “I miss home so much.”

George Yeo, the Foreign Minister of Singapore, said that members of the Association of Southeast Asian Nations (Asean) were concerned about relations between Thailand and Cambodia.

"We are very concerned about this bilateral problem between two fraternal members of Asean and we hope they will find a way to reconcile and to act with restraint," Mr Yeo said.

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