Thailand's former PM Thaksin Shinawatra has been jailed upon returning to the country after 15 years in exile.

But many believe he has struck a deal that will keep him from serving more than a short period in prison.

He arrived in Bangkok on Tuesday morning in a private jet, hours before his Pheu Thai party's candidate Srettha Thavisin was voted the next Thai PM.

This cements Pheu Thai's coalition with its former military rivals who deposed the party in 2014 in a coup.

Mr Thaksin, Thailand's most successful elected leader, has long been feared by conservative royalists, who have backed military coups and contentious court cases to weaken him. He went into self-imposed exile in 2008 after being deposed by a coup two years earlier.

While he made no secret of his yearning to be back in Thailand, what kept him away so long was the various criminal cases hanging over him. But now the brash, politically ambitious telecoms tycoon is back - and was almost immediately sentenced to eight years' jail on criminal convictions he says are politically motivated.

He arrived to cheers from hundreds of loyal "red shirt" supporters who had gathered overnight to see him, but he never greeted most of them.

Samniang Kongpolparn, 63, was among those who had travelled from Surin province in the northeast, the stronghold of Mr Thaksin's party in past decades.

"He's the best prime minister we've ever had. Even though I won't get to see him today, I still wanted to come to show him support," she said. "I'm ok with them reconciling with the pro-military government, or else we're stuck with the senators. We don't want that."

Flanked by his two daughters and son, Mr Thaksin emerged briefly from the airport terminal and paid his respects to a portrait of the king and queen. The 74-year-old was then taken to the Supreme Court where he was sentenced and then to Bangkok Remand Prison.

It has been speculated that Thaksin will seek a royal pardon, and prison authorities on Tuesday said he would be able to submit a petition from jail immediately. The process can take one to two months.

Prison authorities there say he will be kept in a wing with specific medical equipment, given his advanced age. He will also immediately undergo a 10-day quarantine - the first five days of which he will be confined to his room, authorities said.

Thaksin's political party in the front seat

Mr Srettha's win on Tuesday is the result of a byzantine process which in three months has taken Thailand full circle.

It began with the heady hopes of a new dawn led by the radical young Move Forward party, which won the most seats in the May election.

Move Forward initially formed a partnership with Pheu Thai but the coalition now includes almost everyone but the reformers, including two parties led by former coup-makers - a deal with its sworn enemies that Pheu Thai vowed it would not do.

Pheu Thai insists the two developments - Mr Thaksin's return and the coalition that voted for a Pheu Thai PM - are unconnected. Few people believe that.

It is true that Pheu Thai's hands have been tied by the unelected senate, a 250-seat constitutional landmine planted in Thailand's political landscape by the military junta which ruled for five years after a 2014 coup.

And Pheu Thai's bargaining position was weakened by its poorer-than-expected performance in the election, when it lost a lot of support to Move Forward and for the first time was relegated to second place.

The senators, all appointed under the junta, are allowed to join the 500 elected MPs in voting for the new prime minister. Their thinly-disguised remit is to block any party which might threaten the status quo - the nexus of monarchy, military and big business which has dominated decision-making in Thailand for decades.

Unsurprisingly they refused to back the Move Forward-led coalition with Pheu Thai, despite its commanding majority in the lower house. When it was Pheu Thai's turn to negotiate a new coalition, its need for senate support meant it had to take in some of its former opponents.

However some Pheu Thai politicians argue that the party should have held out for a better deal, by refusing to be in a government with the most hard-line conservative groups. Any minority administration formed without Pheu Thai and Move Forward would quickly collapse, because the senators cannot join normal parliamentary votes on issues like the budget.

But the Pheu Thai leadership was not willing to wait. It even invited the ultra-royalist party United Thai Nation to join the coalition, whose leaders have in the past been virulently critical of the Shinawatra family and their supporters. They were instrumental in ousting the last Pheu Thai government led by Mr Thaksin's sister Yingluck. That these two factions will now sit together in the same government is a mark of how far Thai politics has shifted.

In the end, for the ultra-royalists, the perceived threat posed by Move Forward, and by a younger generation of Thais demanding a conversation about the power and wealth of the monarchy, eclipsed their long feud with the Shinawatra family.

For the Shinawatras, and Pheu Thai's more conservative, business-minded elements, getting into government again and guaranteeing the deal to bring Mr Thaksin back, have been bigger priorities than worrying about the party's reputation.

But there are those, even within Pheu Thai, who are horrified by the cynical pragmatism of this deal.

They are warning that the party will lose even more of its once-passionate grass-roots supporters - and lose, perhaps forever, the dominance it held over electoral politics in Thailand for two decades.