"The (BFU's) disciplinary commission imposed a fine of 10,000 levs (4,033 pounds) following an official notification from UEFA for serious doubts about the participation of Marek in league matches with a pre-arranged outcome," the BFU said in a statement on Tuesday.
The BFU were not immediately available for further comment.
Local media said the BFU took the decision after receiving a 45-page report from UEFA, adding that a bookmaker had reported that Marek's away losses to champions Ludogorets (3-0) and leaders CSKA Sofia (5-0) in August had attracted heavy betting.
"We insist on a detailed investigation and we expect concrete evidence to be provided," Marek's chief executive Ivo Pargov told local media.
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"On behalf of the club, I declare that if such evidence is not provided, we'll turn to court because it damages the prestige of Marek."
Marek, the 1978 Bulgarian Cup winners, are 11th in the 12-team domestic league with nine points from 12 matches.
Media reports of match fixing and corruption have been rife in the Balkan country for years.
The first documented manipulated game in Bulgaria took place in 1949, when Levski Sofia and city neighbours Akademik drew 1-1 to "help" rivals CSKA to be relegated to the second division.
(Reporting by Angel Krasimirov, editing by Stephen Wood)
