The Lord Mayor's Mansion House is set to be occupied this weekend - as a group of beaver scouts from Wandsworth hold a sponsored sleepover.
The group of six- to eight-year-olds will camp out in the Great Egyptian Hall, surrounded by 150-year-old statues, to raise money for the Sparks children's charity.
The 15 boys and girls will also get a private tour of the Tower of London by a Beefeater, watching the famous Ceremony of the Keys locking-up ritual before bedding down after a midnight feast.
The Beefeater will also formally "invest" seven of the newer colony members, handing them their scarves after hearing the beaver promise.
It follows a similar sleepover in Downing Street last summer, and for many will be their first night away from home.
They will all earn a specially-embroidered badge featuring a City of London dragon over a silhouette of Tower Bridge, the Gherkin, St Paul's Cathedral, Monument and Tower Bridge.
The Lord Mayor, Alderman David Wootton, will not be disturbed - he is out of the country.
Andrew Cheeseman said his six-year-old son George was "very excited" about the event.
Scout leader Harvey Ward said: "Not many children can say they've had a Beefeater give them a private tour of the Tower of London.
"Hopefully they will have a lot of fun while raising some money and awareness of Sparks. We're planning to hold a series of sleepovers and hope we can inspire other groups to follow suit."
Sparks funds medical research into conditions affecting babies, children and mothers-to-be. The Scouts' latest effort follows several other fundraising events for charities including the Poppy Appeal.
To donate log on to: justgiving.com/Beavers-secret-sleepover.
