The warning was placed on seats prior to Newcastle United’s game against Fulham, threatening that supporters who kept standing up could lose their season tickets.
The note stated: “The club requests that supporters do not persistently stand during matches and sit when requested to do so by stewards and other supporters.
“Any supporters who persistently stand will be identified and sanctions will be taken against them.”
The letter, which was only distributed to selected seats within the stadium, was backed up by an announcement on the Tannoy before kick-off reiterating the rules about persistent standing.