West Ham have banned half-and-half scarves from Saturday's Premier League game with Tottenham.
The Hammers say the unusual move is part of "increased security activity" that will take place at the London derby, which kicks off at 17:30 BST.
It is one of a number of measures the club have taken "for the safety of all supporters and to prevent any fans of the away club from gaining access to home sections of the stadium".
Security staff will confiscate the half-and-half scarves - or any other item featuring the colours or crests of both clubs - before fans enter London Stadium.
West Ham said their increased security presence will include "additional checks at turnstiles, restrictions on half-and-half scarves or items illustrating the colours/crests of both clubs and steps to ensure home areas remain for West Ham United fans only".
"Supporters who have purchased half-and-half scarves from unofficial sellers will be required to hand them over to security staff at the security checkpoints and these scarves will not be returned."
West Ham are based in east London and traditionally, Championship side Millwall have been their fiercest rivals, although the two sides have not met since 2012.
Tottenham's biggest rivalry is with north London neighbours Arsenal but Spurs are arguably the Hammers' biggest rivals in the Premier League.
West Ham have lost three of their opening four games of the season and their official fan advisory board has issued a vote of no confidence in the club's board.
Another fan group has called for a protest, external before the home game against Crystal Palace on 20 September and a boycott of Brentford's visit on 20 October.
German side Borussia Dortmund banned half-and-half merchandise for a Europa League tie against Red Bull Salzburg in 2018, because the Austrian club's badge features the logo of energy drinks manufacturer Red Bull, which owns the club.