Ronny Deila has said that a "lack of improvement" at Celtic, particularly in the last six months, was key to his decision to leave the club at the end of the season.

Celtic announced on Wednesday that the manager would be departing at the end of the current campaign and Deila reflected on the disappointment of recent months as he explained his decision.

Though still favourites to win the Premiership this season, Celtic have been knocked out of both cups and have been criticised for being far from their best in recent months. Deila said that he had become frustrated at being unable to take the team forward and felt that he had to take full responsibility.

"First of all it's been a tough week," he said. "There's been a lot of thinking.

"I've been thinking about this for a little while because I haven't felt the team has been getting the results they should have and, maybe most important, the performance that I wanted or the improvement.

"When I first came into this job I talked about improving the team, about playing attractive football and I think in the last six months we haven't had the improvement that we need. When you don't get improvement, one of my biggest values, then it kills me inside.

"I lose energy over that. That's why you are a teacher, are a manager, because you want improvement in the team."

The Norwegian said that the doubts that began to circulate about his long-term future at the club could have been a factor in lacklustre performances and that was a factor in his decision to step aside.

Deila said that he had enjoyed his time in Glasgow and that if he could retain the league title then he would be able to leave the club on a high.

"Now we can focus on winning the five games and getting the five in a row," he said "It's going to be a huge, huge thing for me and also for the players and the club."