West Ham are on high alert to prevent up to 7,000 rowdy away fans entering the home end for Thursday's European clash.
The Premier League side host Romanian outfit Steaua Bucharest at the London Stadium in their Europa Conference League group-stage opener on Thursday evening.
And amid FCSB president Gigi Becali's claims that 10,000 will rock up despite a ticket allocation of 3,125 - sparking fears away fans could infiltrate opposition sections and sit among Hammers supporters - Sportsmail understands West Ham have readied a raft of measures to prevent it happening.
Their prevention techniques are understood to include seats in the home end being limited to supporters with a previous booking history, and no tickets being made available to customers from Romania even if they have a previous booking history.
FCSB fans attempting to enter the home end will not be allowed in, and if any fans are spotted inside they will be promptly ejected.
Becali said: 'I think there will be over 10,000 Romanians with West Ham. There were long queues for tickets today.'
Steaua announced on Wednesday night they had sold out the entire allocation of 3,125, which is the maximum number of tickets West Ham could offer to the visitors.
On Wednesday, thousands of FCSB fans flocked to the Hammers' ground in a bid to pick up a ticket, starting to queue from around 9am BST - more than eight hours before the ticket shop opened at 5pm.
At that time, the queue was huge, with social media footage showing fans snaking around the stadium for 300-400metres.
The delay was reportedly due to uncertainty about how to sell the tickets, with the conclusion being each fan could buy two tickets each.
The London Stadium in Stratford, which was designed for the 2012 Olympic and Paralympic Games in London, has a capacity of 62,500.
Last season, ahead of West Ham's Europa League semi-final clash with Eintracht Frankfurt, German fans flocked to the stadium in huge numbers and were escorted by the police.
There was a row over tickets following a threat from West Ham to ban any supporter found to have sold their seat to an away fan after an estimated 30,000 Frankfurt backers went to the Nou Camp for their quarter-final tie with Barcelona.
The Bundesliga side might not have been able to bring as many as that to east London, but they came in sizeable numbers to create a huge atmosphere along with the Hammers.
