AMSTERDAM, Dec 15 (Reuters) - Crown Princess Amalia, the heir to the Dutch throne, invited 21 people to her 18th birthday party last week even as the government has asked people not to have more than four guests in the face of surging COVID-19 cases. Amalia, who had cancelled indoor festivities because of the pandemic, held a last-minute gathering in the palace gardens, Dutch Prime Minister Mark Rutte wrote to parliament on Wednesday. The eldest daughter of King Willem-Alexander had been expected to quietly celebrate her birthday last Tuesday. read more . Although no rules appear to have been broken, the event came as hospitals struggled to find beds for a surge in COVID-19 patients and concerns about the new Omicron variant. Social-distancing restrictions were in place nationwide, including the early closing of restaurants and non-essential shops. Professional sporting events were closed to the public and households had been asked not to have more than four guests. read more "Guests were asked to take a test. All were vaccinated. The assumption was that they would keep an acceptable distance," Rutte"s letter to lawmakers said in reference to the party. But King Willem-Alexander "informed me that in hindsight, it was not such a good idea. I think that"s a sensible reaction," Rutte wrote. The public disclosure of a usually private royal affair came days after British Prime Minister Boris Johnson faced pressure over assertions that his communications chief attended a festive gathering at his official Downing Street residence during a lockdown last year. read more In October 2020, the Dutch royal family cut short a vacation to its holiday home in Greece after being criticized for travelling during the pandemic while the general population had been advised against going abroad.