Touts can sell tickets for many times their face value
Police are warning that invalid tickets will be on sale from touts outside the Chelsea ground for Sunday's game against Liverpool.
Scotland Yard said a batch of tickets for the match at Stamford Bridge in south-west London were bought illegally and would be refused at the turnstiles.
Officers said that even if someone managed to get in with the tickets they would find the seats already taken by genuine ticket-holders.
Police have advised anyone who has doubts about their tickets to check with stewards at the ground.
The match is a crunch game with both teams battling to qualify for Europe.
The place in the Champions League is worth up to £15m to a club.