Euro 2016 matches could be held behind closed doors this summer due to the terror threat in France.
Uefa executive vice-committee president Giancarlo Abete has admitted that locking fans out of game could be a possibility if their safety cannot be garuanteed.
Following two blasts in Brussels on Wednesday which left more than 30 dead, Uefa issued a statement insisting that the tournament was still going ahead as planned with "safety and security at the centre of its organisational plans".
But Abete has said more stringent measures may need to be implemented.
"Euro 2016 is the kind of event we can't delay or postpone," he told Radio 24. "We can't exclude the possibility of playing behind closed doors as we cannot exclude terrorism.
"If we talked about potentially cancellable games such as a friendly or a competitive match that could be moved to another date, obviously this would not be the case.
"But we are talking about games which are staged for June, whereas today we are going through very urgent emergency."
Euro 2016 organisers last month said that 10,000 people had been employed in security roles including 900 guards for each of the 51 matches.
In a statement last night Uefa said:
“We will continue their joint work and will regularly monitor the level of risk for the tournament and their respective organisational plans.
“For over three years now, Euro 2016 SAS (safety and security) has been working closely with the relevant authorities to develop the most appropriate mechanisms in order to guarantee a safe and secure tournament and all necessary measures are being taken to ensure that is the case for all involved."