Tonev's move includes an option for Celtic to buy the 24-year-old at the end of the season.

He agreed the switch to Parkhead after speaking to compatriot Stiliyan Petrov, who enjoyed a successful seven-year spell at the Glasgow club before joining Villa in 2006.

"Joining Celtic is unbelievable and I feel really good about it. I can't wait to meet up with my new team-mates and start playing for Celtic," Tonev told the club's website.

"I already knew all about Celtic. Everybody in Bulgaria knows Celtic.

"I spoke to Stiliyan Petrov yesterday and he was very happy for me. He wished me good luck and I hope I can do as well for Celtic as Stiliyan did."

Tonev has won 15 caps for Bulgaria and Villa hoped he would be a key figure when Paul Lambert signed him from Polish side Lech Poznan in June 2013.

Petrov, who was on the Villa coaching staff at the time after an earlier battle with leukaemia, predicted his countryman could make a substantial impact, but Tonev struggled to make an impression in the English top-flight.

However, Celtic manager Ronny Deila believes Tonev has more to offer than he showed in England.

"Aleksandar's a very quick player and he has two good feet which he can shoot with," he said.

"He has scored a lot of goals during his time at Lech Poznan and I think he can help bring the club forward with his qualities. We can also make him better."