By Ed Cropley, Reuters
December 5, 2008 - 12:00 a.m. EST

BANGKOK (Reuters) - Thailand's revered and influential king missed his traditional birthday eve address due to illness on Thursday, a speech many Thais had hoped would provide short-term relief to the country's intractable political crisis.

King Bhumibol Adulyadej, who turns 81 on Friday, has a throat infection and is on a drip, his daughter said in remarks aired on national radio from Bangkok's Chitralada Palace.

Many Thais had been looking to the world's longest reigning monarch to issue a call for unity after the political maelstrom saw Bangkok's main airport shut for a week by royalist People's Alliance for Democracy (PAD) protesters.

"Yesterday when I saw him he looked OK. He could eat what we served him, but today he had a throat infection so he could barely eat," Crown Princess Maha Chakri Sirindhorn said.

"Therefore, the doctors prescribed him medicine and put him on a saline drip," she said.

Thailand's caretaker government canceled next Monday's special parliamentary session to choose a successor to Somchai Wongsawat, banned from politics for five years for voter fraud.

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

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.

Crown Prince Vajiralongkorn, who also deputized at the ceremony, said his father was "slightly ill." Neither the prince nor the princess made any mention of politics.

Well-wishers rushed to pray at a statue of the king's late father, Thai media said. Bhumibol spent several weeks in hospital last year with circulation problems and there are growing concerns about his health.

The monarch has been thrust into the center 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.

HOPE FOR TOURISTS

Bringing hope to 230,000 stranded foreign tourists, Airports of Thailand said the $4 billion Suvarnabhumi airport, one of Asia's largest, would resume "full service" at 0400 GMT on Friday after a week-long shutdown by PAD protesters.

Thai Airways said it had 12 flights out on Thursday but sources said other carriers were being railroaded into getting back in the air and were worried about the effect of short-cuts on safety and security procedures.

"We are under enormous pressure to open -- from the airport authorities, from stuck passengers, from shareholders, from the tourist industry," said one airline official who asked not to be named. "But our genuine security concerns are being ignored."

The airport shutdown has already cost the tourism- and export-dependent economy hundreds of millions of dollars. The central bank cited the economic impact of the unrest when it cut interest rates by one point to 2.75 percent on Wednesday.

Exports are already feeling the pinch from the global slowdown, and Moody's followed other rating agencies in cutting its outlook for Thailand to negative from stable, warning the political problems could undermine government creditworthiness.

MORE TROUBLE IN STORE

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.

The caretaker government called off a special parliamentary session on Monday to select a replacement for Prime Minister Somchai Wongsawat, sacked by the courts this week.

Speaker Chai Chidchob told reporters the king had not responded to parliament's request for an extraordinary sitting. His comments came before news of the king's illness.

Somchai's People Power Party (PPP), which the PAD accuses of being a front for the now-exiled Thaksin, was dissolved in the same ruling but most of its rank-and-file members simply switched to another "shell" party.

It and the other five parties in the ruling coalition easily have the numbers in parliament to form the next administration, an eventuality that seems bound to lead the PAD to resume its street protests.

(Additional reporting by David Fox; editing by Philippa Fletcher)