Celtic could be handed a sensational Champions League reprieve after UEFA launched an investigation into the eligibility of one of Legia Warsaw's players during their 6-1 aggregate win over the Scottish champions.
The Polish side confirmed that they had been asked for information from the European governing body.
A statement on Legia's official website read: "The club announces that in connection with the Celtic FC-Legia Warsaw match in the second leg of the third qualifying round of the Champions League, UEFA opened an investigation into the participation of Bartosz Bereszynski.
"The club is preparing the relevant information and explanations, which will be sent to UEFA today.
"The result of the investigation will be announced immediately after its completion."
Bereszynski was sent off for violent conduct against Apollon in the final match of Legia's Europa League campaign last season.
The full-back missed both of Legia's matches against St Patrick's in the second qualifying round and sat out the 4-1 first-leg victory over Celtic.
But the investigation apparently centres around whether the player was registered in Legia's squad against the Irish side in order to allow a three-match suspension to take effect.
Bereszynski only came on in the 86th minute of Legia's 2-0 win at Murrayfield, when the game was well and truly over, but the context and impact of his appearance might not have any influence on the outcome of the investigation if rules were breached.
Champions League regulations state that players must be registered with UEFA within requested deadlines and that "only eligible players can serve pending suspensions".
UEFA's disciplinary regulations decree that a match may be declared a forfeit if an ineligible player participates, "as long as the opposing team files a protest".
The team forfeiting the match is deemed to have lost 3-0, which would put Celtic through on the away goals rule.
The UEFA administration decides on eligibility questions while any appeals are dealt with by UEFA's control and disciplinary body.
Celtic benefited from an ineligibility ruling in 2011-12 when they were reinstated into the Europa League group stages despite a qualifying defeat by Sion.
The Swiss side were ruled to have fielded five ineligible players after breaching a FIFA transfer ban and failed to reverse the decision during a lengthy legal battle.