Wigan Athletic have so far failed to sell at least 10,000 of their allocated tickets for Saturday's FA Cup semi-final against Millwall, reports BBC Sport. Both clubs were allocated just over 31,000 tickets for the game.
While the Championship club are expected to sell their full complement, Wigan are struggling to sell their section.
A maximum of 6,000 tickets from what was originally a Wigan section will now be sold to neutrals before Saturday's game.
But even with sales in the neutral zone, it is understood the authorities expect a minimum of 14,000 empty seats at Wembley.
Wigan Athletic declined to give details of exact ticket sales when asked.