Millwall take on Dagenham and Redbridge in the FA Cup on Monday and there are growing concerns that a number of West Ham fans have snapped up tickets for the away end at the Den.
The Hammers have not faced their old foes since 2012 and there have been, perhaps surprisingly, no restrictions on who can buy tickets.
Last week, more than 1,300 had been sold by Dagenham and Redbridge, the National League side with close links to West Ham, and Met Police — who deemed the match low risk — are aware of the situation.