The Princess of Wales and Princess Charlotte were reportedly spotted travelling on the Elizabeth Line during a low-key trip into central London this weekend.
Kate and her 11-year-old daughter took one of Transport for London's busiest rail services from west London to Tottenham Court Road, blending in with fellow passengers during the journey.
Footage shared online appeared to show the pair travelling alongside members of the public with a discreet security presence, rather than using the official transport often associated with royal engagements, The Sun reports.
A fellow passenger told the newspaper the sighting was "surreal".
"It was really unexpected to see the Princess of Wales and Princess Charlotte on the Elizabeth Line," the passenger added.
The reported outing is believed to be one of the rare occasions a senior royal has been seen using London's public transport network outside an official engagement.
Neither Kensington Palace nor TfL has commented on the reported journey.
The pair are understood to have disembarked at Tottenham Court Road, placing them within walking distance of some of central London's most popular attractions, including the British Museum, the West End and Oxford Street. The purpose of the trip has not been disclosed.
The Elizabeth Line, named in honour of Queen Elizabeth II, opened in 2022 and has become one of the capital's busiest transport routes, linking east and west London through central stations including Tottenham Court Road, Bond Street and Liverpool Street.
While members of the Royal Family often travel by car as part of official security arrangements, Kate has previously spoken about trying to give her children as normal an upbringing as possible despite their position within the monarchy.