Thais marked their revered king's birthday today in a solemn mood, concerned for the health of the ageing monarch and worried as well over their country's debilitating political deadlock.

King Bhumibol Adulyadej, who turned 81 today, failed to deliver his annual birthday eve address at a time when many in Thailand had been looking to him to issue a call for unity after protests by thousands of 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.

Thailand's caretaker government cancelled next Monday's special parliamentary session to choose a successor to Somchai Wongsawat, Mr 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 the Australian National University (ANU), said.

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

Bringing hope to 230,000 stranded foreign tourists, Airports of Thailand said the billion Suvarnabhumi airport, one of Asia's largest, resumed full operations today 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.

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.

After the caretaker government called off a special parliamentary session on Monday to select a replacement for Mr Somchai, 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.

Mr Somchai's People Power Party (PPP), which the PAD accuses of being a front for the exiled Thaksin, was dissolved by the courts this week 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.