Brussels - It's set to be a royal moment: for the first time since Belgium declared independence in 1830, members of the royal household will now have to pay taxes.

Backed up by two parties in the opposition, the ruling coalition on Wednesday approved the reform.

King Albert II, who turns 79 on Thursday and who enjoys an annual tax-free allowance of 11.5 million euros (S$18.8 million), will be exempt however, because the constitution does not allow changing rules for the reigning royal.

Instead, it is the king's children and the widow of king Baudouin, Queen Fabiola, who will be affected by the new rules.

Under the current system, 53-year-old throne-heir Prince Philippe receives around 923,000 euros a year, but from now on he will instead receive a gross salary of 180,000 euros, subject to taxes.

Money earmarked for staff and expenses related to carrying out his job will be taken from the initial stipend and will not be taxed, but those costs will be closely scrutinised.

His siblings, Princess Astrid and Prince Laurent, will receive gross wages of 90,000 euros.

Queen Fabiola, whose annual allowance was this year slashed from 1.4 million euros to 923,000 euros, recently faced an unprecedented attack from the media, the public and politicians after deciding at the age of 84 to set up a private foundation to provide funds for her nephews and for Catholic charities on her death.

Critics accused the Baudouin's widow of planning to funnel the funds to Spanish relatives via the foundation, thus avoiding Belgium's 70 per cent death duties.

From now on, the surviving spouse of a sovereign will have funds cut to two-thirds of that of an heir for 10 years, and to just half of it beyond that period.

In the future, only the heir of the throne will receive an allowance.