Commons passed a medical and agreed personal terms with the Hoops yesterday before the finishing touches were applied to a three-and-a-half-year deal this morning.

Commons was out of contract at the end of the season and is understood to have cost Celtic in the region of £300,000.

The club have an option to extend the 27-year-old's initial deal by an additional year.

Celtic boss Neil Lennon, a former Nottingham Forest team-mate of Commons, won the battle for the player's signature despite interest from Old Firm rivals Rangers.

Gers boss Walter Smith was keen on Commons but could not match Celtic's package financially.

Derby chief executive Tom Glick issued a statement to his club's official website explaining the decision to sell Commons, who had rejected a new deal after lengthy talks.

"Our discussions with Kris over a new contract began back in October and he made it abundantly clear he wanted to stay at Derby County," explained Glick, speaking to dcfc.co.uk.

"Kris stated this publicly in the local media as well, so the fans were hearing the same messages as us.

"Likewise, we made it very obvious we wanted him to stay and build on the success he has had in a Derby County shirt.

"When we began discussions with Kris, he gave us a figure in terms of wages and length of contract that he was looking for.

"After lengthy negotiations with Kris' agent, we put an offer on the table to match exactly what Kris requested.

"Two weeks ago we thought we were almost done and dusted because we had a productive meeting and made the offer that Kris had requested.

"By this point however, it became apparent that Kris wasn't going to sign the contract that was on offer to him.

"We then decided, as a club, that it was in our best interests to accept Celtic's offer to sign Kris now for an undisclosed fee.

"It is disappointing to lose Kris, he is a very talented footballer and we always wanted him to stay.

"We did everything we could to make him stay and we offered a very good deal to him."

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Lennon told Celtic's Channel 67: "Kris is a goalscorer.

"I've known him for three or four years from my time when I spent six months at Nottingham Forest.

"I always liked his style, he can play either side of midfield and he can play off the striker.

"He's predominantly left-footed but he can hit a good ball with his right foot so he's got good feet.

"He's a clever player. He's sharp and quick around the box and he has scored 13 or 14 goals this season for Derby from a deeper position.

"What he brings is a bit of extra quality in the final third. We know he's a goal threat from any position he plays.

"I like his temperament as well. He never gets disappointed, he likes to keep going."

Celtic face Aberdeen in the last four of the Co-operative Insurance Cup this weekend.

Lennon added: "He will be in the squad for the semi-final. Whether he starts, he will definitely be in contention."