http://news.bbc.co.uk/sport1/hi/football/teams/c/celtic/8896037.stmAiden McGeady has completed his £9.5m switch from Celtic to Spartak Moscow and agreed a four-and-half-year contract with the Russian club.
The 24-year-old Republic of Ireland midfielder is the most expensive ever export from Scottish football.
"Celtic is the team I have supported my whole life and I will miss so many things," McGeady told the club website.
"However, now I am looking forward to the new challenge of playing for Spartak Moscow."
McGeady made his Celtic debut in April 2004 when he scored in the 1-1 draw against Hearts at Tynecastle and went on to make 252 appearances, scoring 37 goals.
He won three league titles, as well as two Scottish Cups and two League Cups.
"It's what he wanted to do and sometimes you can't stand in the way of that," said Celtic boss Neil Lennon on Thursday.
"It's a huge offer and I think it's the right offer. It's probably good business for Spartak and good business for us."
McGeady's departure will provide Lennon with funds to bolster his squad and may see Celtic revive their interest in Stoke winger Liam Lawrence.
The money generated from the record transfer could also help facilitate a loan move for Manchester City forward Craig Bellamy.
However, the manager expressed sadness at losing such a talented player.
"I'm sorry to lose him, he is one of the entertainers in the game," said Lennon.
"We'll probably miss out on his peak years but he came to us and said that he felt at the end of the season it would be better for him to move on.
"It's difficult to stand in the way of someone who doesn't want to be here."
One player hoping to benefit from McGeady's exit is Pat McCourt, a winger who has been on the fringes of the first team for two seasons now.
"We've probably lost the best player in the league," said the Northern Ireland international of McGeady.
"I'm sure it's going to be a big hole. But it's up to someone to step up to the plate and fill it.
"It's up to whoever gets a shout and gets the jersey to hold on to it, and I'm hoping it'll be me. This year's the year I really want to kick on and be a regular in the side."
Spartak, managed by former Russian World Cup star Valery Karpin, lie in eighth position in the Russian Premier League, having won only five of their 15 matches.
All the best wee mhan, thanks for the memories
