Queen Elizabeth allowed 500 Windsor Castle staff and their pals to put on an Abba tribute party.
The 91-year-old monarch - who is currently on holiday in her Scottish residence of Balmoral - is said to have given the green light to the bash at her royal residence in Berkshire, south east England, for which staff and their friends paid £12.50 each.
A Royal source told the Mail on Sunday newspaper: "There are staff events for those who work here. I am sure there will be similar events in the future."
The event was put on by the Windsor Castle Cinema Club, which was set up by the royal staff to show films at the historic venue.
At their latest bash, an Abba tribute band played followed by the screening of hit musical movie 'Mamma Mia!', which features several of the group's songs and is named after one of their massive tunes.
Royal staff are said to have put the names of their guests down on a list to be checked by security, and attendees admitted the event went off with great success.
Erika Baker, 48, a tanning salon owner from Windsor, told the publication: "Members of the Royal Household were allowed to buy tickets for themselves and guests.
"It meant lots of ordinary people from Windsor could come in for the night.
"Everybody was very well behaved, and once you were inside you were encouraged to really let your hair down and dance and sing."
A Palace spokesman said: "We would not comment as this was a private event."
In May, the queen was said to be considering a move to Windsor Castle later this year while an extensive refurbishment is carried out at Buckingham Palace.
A royal butler said at the time: "What is in the pipeline is Buckingham Palace has got to get a refurb. I think what we will do is use that - not as an excuse - but a reason for the queen to move to Windsor. It will be after her summer break at Balmoral Estate - when she comes back in October - that's when it could happen. That's if does happen this year though. If it does it will be then."