The Windsor Hotel has fallen out of favour with its younger clientele after raising the minimum entry age to its garden bar without notice.

The South Perth pub made the sudden decision to ban patrons aged under 25 from its back bar after anti-social behaviour from some young punters, including a group called "Tommy and his crew".

The historic Windsor Hotel in South Perth. Credit:Facebook

All other areas in the hotel are open to people aged 18+.

A representative from the Windsor said the pub was forced to take "drastic" action to get the message across to the group of young men, who are believed to be from southern WA and other remote locations around the state.