In Belgium's papers, King Albert II has been portrayed by the country's most fabled cartoonist for the past 20 years as just out of bed and lounging about in a dressing-gown and slippers.

Cartoonist Pierre Kroll's portrayal of a jovial homebody, along with Albert's legendary love of jokes, "helped people to like him and brought him closer to the people," said Bernard Marliere, author of a book on Belgian humour.

But the easy-going style of the monarch, who abdicates Sunday in favour of his son Philippe, belies the power wielded by the sovereign in a country split by language, religion and history.

Belgium heads next year for a high-risk May 25 general election that could see huge gains for the powerful Flemish separatist party, the N-VA.

"Some parties ... want to rip our country up. They'll have me to deal with, I can be fearsome, I won't take anything lying down," Philippe said in 2004 in remarks that upset the affluent northern region of Flanders.

Yet commentators and French-speakers alike fear that 53-year-old Philippe, said to be as shy and awkward as his father is relaxed and friendly, may not share Albert's political nous.

The N-VA's republican-minded leader Bart De Wever, powerful mayor of the flourishing seaport city of Antwerp, thumbed his nose at the monarch not so long ago, arriving at the royal palace for talks without a tie.

"I'm a republican because I'm a democrat. I don't believe in someone having power thanks to their birth," De Wever said.

The House of Belgium has ruled the small country of 11.5 million people at the crossroads between the Latin and Germanic worlds since gaining independence from The Netherlands in 1830.

Its Flemish-speaking northerners at the time were down-at-heel, the French-speakers lording it on the land and later over the industrial belt in the south. The tables since have turned.

De Wever's followers deride the Socialist-leaning French-speakers of southern Wallonia.

Though the king "reigns but does not rule", being from neither camp, Albert played the key role of political umpire through two decades at Belgium's helm, steering the country skilfully away from crisis and potential break-up.

So many hoped to see him remain on the throne until next year's elections in case of strong gains by the separatist N-VA.

But after a 2010-2011 political crisis that left Belgium without a government for 541 days, others said the king hoped to give his heir time to make his mark and stake out the ground before the polls.

"The abdication will enable the future king to take on the mantle and to meet political leaders ahead of the elections. Everyone fears a repeat of the 2010-2011 crisis," said political scientist Caroline Van Wynsberghe of Brussels' ULB University.

Former royal counsellor Pierre-Yves Monette said Philippe is likely to play a far bigger political role than his peers in Europe's other royal houses.

"The king is supposed to be the guarantor of federal loyalty, and therefore neutral," Monette told AFP.

"He does a lot more than open flower shows though," he said, adding that by handing the mandate to form a government to a political leader he "plays a central role."

Through the 2010-2011 impasse, Albert successively gave a string of party leaders the mandate to try to form a federal coalition, now in the hands of Socialist Prime Minister Elio Di Rupo.

Former Belgian premier and current European Union president Herman Van Rompuy said that Albert "changed the game" with his management of the crisis. "And the country didn't implode," Van Rompuy said.