Prime Minister Prayuth Chan-ocha's new United Thai Nation party will be challenged mainly by the Pheu Thai party, led by former PM Thaksin Shinawatra's daughter Paetongtarn.

Thaksin, a billionaire, was deposed by a military coup in 2006.

Paetongtarn, 36, has led Prayuth in opinion polls for months.

A date for the election has not been set, but it must take place within 60 days of dissolution.

But campaigning by dozens of parties is already under way. Bangkok's pavements are disappearing behind a blizzard of party posters making all kinds of promises to the voters.

In the end, though, this election is really about one thing: can Pheu Thai win by a sufficiently large margin to ensure it takes power again? Nearly every poll is predicting that it will once again be the largest party, as it has been in every election for the past 22 years, relying on strong loyalty to Mr Thaksin in the north and north-east.

Some think it may even win an outright majority of seats in the lower house. But that may not be enough, given the enduring animosity towards him and his allies from conservative royalists and the military.

In the past, judicial rulings or military coups have prevented three Thaksin-backed governments, including one led by his sister Yingluck, from completing their term.

Prayut, a retired general, has been in power since leading a coup against Ms Yingluck's government.

Thaksin has been in exile for 15 years, avoiding a list of criminal charges, even as many of his lieutenants are now banned from politics.

Yet he is still there, hovering over this election like a ghost, his daughter being the latest Shinawatra family member to front the party.

Speaking on Friday at an event to introduce Pheu Thai's candidates, Ms Paetongtarn said she was confident of winning the election by a landslide.

After the last coup, the military resolved to finish the Thaksin problem once and for all by rewriting the constitution to ensure his party could not take power. They appointed 250 senators, most of whom presumed still to be loyal to Generals Prayuth and Prawit Wongsuwan, the men who led the last coup.

With the senators' backing, and after a good deal of manoeuvring, Pheu Thai was kept out of office at the last election in 2019. The two generals have led a fractious conservative coalition since then.

However they now each head their own parties, at risk of dividing the conservative vote.

Under the military-drafted constitution, the senators can still vote one more time on the choice of the next prime minister. With their support, the two generals could still form a government even if Pheu Thai does win a majority.

But the senators cannot vote for laws or budgets, and any administration which depends on their backing cannot function. If Pheu Thai gets more than 200 out of the 500 seats being contested, it will be difficult or even impossible to exclude them from the next government.

This being Thailand, no-one can rule out another extra-parliamentary move against the party; not a coup this time, but perhaps another party dissolution by the reliably conservative courts.