UPDATE 3-Politics on hold as Thais pray for ailing king

(Updates with statement on king's health, para 4)

By David Fox

BANGKOK, Dec 5 (Reuters) - Thais marked King Bhumibol Adulyadej's 81st birthday on Friday in a solemn mood, concerned for the health of the ageing monarch and worried as well over Thailand's debilitating political deadlock.

The king failed to deliver his annual birthday eve address at a time when many Thais had been looking to him to issue a call for unity after protests by People's Alliance for Democracy (PAD) activists shut down Bangkok's main airport for a week.

His speeches in the past three years have been nuanced and focused on the need for national unity, although his calls for clean government were widely read as a swipe at Thaksin Shinawatra, the populist prime minister ousted in a 2006 coup.

A palace statement on Friday said the king had had an intermittent fever since Wednesday but was being treated by doctors at his Bangkok palace. He had been put on a saline and glucose drip because a throat infection stopped him from eating.

Thailand's caretaker government cancelled next Monday's special parliamentary session to choose a successor to Somchai Wongsawat, Thaksin's brother-in-law, who was convicted of vote fraud this week and banned from politics for five years.

Seen as semi-divine by many of Thailand's 65 million people, the king has intervened in politics three times during his six decades on the throne, variously favouring elected and military administrations.

The king spent several weeks in hospital last year with circulation problems and there are concerns about his health.

"If the king is very seriously ill, and we don't know that, it will put the succession issue at the forefront of people's minds," Craig Reynolds, a Thai historian at Australian National University, told Reuters.

The monarch has been thrust into the centre of the fray by the PAD's persistent use of his name in their fight with Thaksin, whose popularity with rural voters, based on cheap healthcare and credit, upset Bangkok's old royal and military elite.

BUSINESS AS USUAL

Bringing hope to 230,000 stranded foreign tourists, Airports of Thailand said the $4 billion Suvarnabhumi airport, one of Asia's largest, resumed full operations on Friday after the week-long shutdown.

"We are up and running and its business as usual," transport minister Santi Prompat told reporters during a tour of the airport on Friday.

The airport shutdown has already cost the tourism- and export-dependent economy hundreds of millions of dollars.

Despite the return of relative normality at the airport, analysts said more trouble was in store after the brief hiatus of the king's birthday.