The Dutch royal family has expressed its regret for inviting 21 people to celebrate the future heir's 18th birthday party despite Covid-19 guidelines being in place.

       Crown Princess Amalia held a gathering in the palace gardens last Saturday.

       The eldest daughter of King Willem-Alexander was expected to quietly celebrate her birthday amid a surge in Covid-19 cases and concerns over the new omicron variant. People in the Netherlands are presently allowed to receive a maximum of four guests over the age of 13.

       Dutch prime minister Mark Rutte, in a letter to the Parliament on Wednesday, wrote: "Guests were asked to take a test. All were vaccinated. The assumption was that they would keep an acceptable distance".

       Mr Rutte added that King Willem-Alexander "informed" him "that in hindsight, it was not such a good idea".

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       "I think that's a sensible reaction," the prime minister wrote.

       Heir to the Dutch throne Princess Amalia, left, takes an honorary seat at the Council of State

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       Although the prime minister's letter did not mention the number of attendees, the local media quoted the government saying 21 invites were sent out for the gathering.

       Earlier in October 2020, the Dutch royal family was criticised for being away on a vacation to their holiday home in Greece during the pandemic.

       The Dutch prime minister on Tuesday announced that the Covid-19 restrictions would be extended through the Christmas holidays.

       The spread of highly contagious omicron variant of the coronavirus is making up roughly 1 per cent of new infections in the country and is a reason to be concerned, Mr Rutte said.

       Elementary schools will close a week early to try to prevent children from infecting older family members, he announced. The prime minister added that a second Christmas "during which grandparents couldn't hug their grandkids under the tree" was a painful necessity.

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       Other restrictions, including the closing of non-essential stores, restaurants, bars and other public places from 5 pm to 5 am were extended through 14 January.

       The Netherlands has registered over 2.9 million coronavirus infections and more than 20,000 related deaths.