Championship club Millwall were given the highest-possible terror alert level last month over fears that The Den could be targeted, according to a report.
Officials are said to have become wary after a staff member who had converted to Islam sent mobile phone footage of the Lions' home to a friend in Pakistan.
The worker was subsequently suspended and counter-terror police were called in, according to The Sun, with the threat level being increased to the maximum level for any football club.
A source is quoted by the publication as saying: "The alert was raised by a member of staff and the club was quick to head off any possible threat.
"The police took it really seriously and plain-clothes officers have been trawling the stadium speaking to staff. Security was stepped up across the club but police have said there's no direct threat."
The member of staff in question, who is understood to be middle aged and has worked at the club for more than a decade, has been spoken to by police but not arrested.