A report that was two years in the making shows that the royal family is a much heavier burden on the public purse than is officially acknowledged, according to the Dutch republican society Republikeins Genootschap.

The official figure, which is made public every budget day, hovers around the €60m mark but, basing itself on anonymous civil servant sources and whistle blowing site ‘Willileaks’, republicans claims the true costs are nearer €350m. That would make the Dutch royal family by far the most expensive royal family in Europe.

The society says there are many hidden costs, such as the upkeep of palaces, and the fact that the royal family don’t pay rent which should  be taken into account.

Many embassies, moreover, have special rooms to accommodate royal guests that stand empty most of the time. Security costs are not in the public domain but, says researcher and editor of the Republican magazine René Zwaap, some €40m is spent each year on keeping the king, his wife and his mother safe.

The main is reason the royals cost so much, however, is that they hardly pay any tax over their €12bn fortune.

In a reaction to broadcaster NOS a spokesperson for the royal family said the report was based on ‘conjecture’ and ‘factually untrue’ because the king does pay tax. The royals do not give out any information regarding their private fortune.

Zwaap himself says in the report that he has used ‘creative methods’ to sustain his claims but that if should they prove false ‘the royal family is welcome to correct them.’