Taxpayers will pick up a £2million security bill for Eugenie’s wedding.
The minor royal will get married at the same venue as Harry and Meghan.
Labour’s Chris Williamson said: “It really is an outrage.”
Security expenses for Princess Eugenie’s wedding have more than doubled amid increased terror fears since she announced her engagement in January, sources say.
Extra firearms officers may have to be drafted in when the ninth in line to the throne weds tequila brand ambassador Jack Brooksbank on October 12.
Eugenie, 28, who carries out no royal duties, has nevertheless chosen to marry at St George’s Chapel in Windsor Castle where Prince Harry wed Meghan in May.
Dad Prince Andrew was said to have demanded a similarly glitzy event for his younger daughter with Sarah Ferguson.
A royal source said: “The Duke of York has insisted his daughter’s wedding must be a huge occasion to be remembered and wants everyone to get on board to celebrate the happy couple.”
Despite warnings about the extra security issues it would cause, Eugenie and Jack are determined to enjoy all the pomp of an open-top carriage procession down Windsor High Street.
It means dozens of officers must be deployed to search the route in advance.
Drones will also be banned from flying over the castle for the whole weekend.
The same tactics were employed for the wedding of the Duke and Duchess of Sussex, but Eugenie’s wedding was initially set to cost around £750,000.
The increased expenses are due to officers’ overtime, cancellation of holidays and the need for extra patrols.