The display of a giant anti-poppy banner by a group of Celtic fans at Parkhead this weekend has been called "regrettable" by Scottish Premier League chief executive Neil Doncaster.
Fans among a section of supporters called the Green Brigade unfurled a banner during half-time of the 9-0 win over Aberdeen which read: "Your deeds would shame all the devils in Hell. Ireland, Iraq, Afghanistan. No bloodstained poppy on our Hoops."
Doncaster said the SPL are investigating this incident, and crowd trouble at the Hibernian v Hearts derby where players were pelted with coins and flares thrown onto the pitch.
The SPL chief executive told Press Association Sport: "It's clearly regrettable that a weekend of so much excitement on the pitch has been dominated by reporting today of events off the pitch.
"We have invited comments from Celtic, Hibernian and Hearts on the back of the weekend's events and are awaiting the reports of the match delegates."
Celtic have already apologised for any offence caused and said they had launched an investigation into the incident, which would appear to breach stadium rules forbidding political demonstrations.
The charity Poppy Scotland has also described the banner as "disrespectful".
A section of Celtic fans have long campaigned against the wearing of poppies but Doncaster said it was now an established practice in the SPL for all clubs to wear poppies on their shirts on the weekend of Remembrance Sunday.
He added: "We would welcome the fact that Celtic have moved quickly to apologise for any offence that has been caused. I would also point out that all 12 of the SPL clubs will be wearing poppies on their shirts next weekend in support of Poppy Scotland for the third year in a row."
Doncaster would not be drawn over whether the banner did breach political rules but added: "There have been a number of instances of inappropriate behaviour at games, these incidents do need to be investigated and they are going to be."
Celtic responded to the controversy by issuing a statement on the club's website which said: "The actions of this small minority have no place at Celtic Park.
"We are currently investigating the matter and, clearly, we apologise for any offence caused."
Some fans of the club, which has a strong Irish Catholic heritage, refuse to support the Poppy Appeal because of the legacy of the Troubles in Northern Ireland, while the Green Brigade are known for demonstrating support for Irish republicanism at games.