Leigh Griffiths was hoping for a fresh start under new Celtic manager Ange Postecoglou
Striker Leigh Griffiths has been sent home from Celtic's training base in Newport while allegations of improper online behaviour are investigated.
It is understood the allegations surround claims that the 30-year-old Scotland cap sent private online messages to an underage girl.
Police Scotland says it has been made aware of the allegations and will investigate.
A Celtic spokesman confirmed they have been "made aware of online comments".
"The club will now look into this matter carefully," he added. "Leigh has left the club's training camp as we do this."
- Can Griffiths take final Celtic chance?
Griffiths, who joined Celtic from from Wolverhampton Wanderers in 2014, this month signed a new one-year contract extension with the Scottish Premiership club.
He had not been a regular starter under Neil Lennon then caretaker manager John Kennedy last season having been criticised by the former for his physical condition on his return after lockdown.
Griffiths was also left out of the Scotland squad for the Euro 2020 finals.
However, a new Celtic contract was announced as his previous one expired on 1 July, with new manager Ange Postecoglou saying he and Griffiths had "positive conversations" about what he expected from the striker in the coming season.
Celtic flew to Wales on Monday for their pre-season training camp at Dragon Park – the country's national football development centre.
They had already announced that Scotland winger James Forrest was unable to take part because he had been identified as a close contact of a positive Covid-19 case.
The news of Griffiths' departure comes hours after Celtic defeated Sheffield Wednesday 3-1 in their first pre-season friendly at Dragon Park, with the striker not among the 22 players who took part.
They face another English League One side, Charlton Athletic, there on Saturday before meeting Bristol City at the Championship club's training centre on 14 July.