Portuguese giants Benfica have completed the signing of Belgium international Axel Witsel.

Witsel, 22, joins from Standard Liege for an undisclosed fee and has signed a five-year deal including a €40 million buyout clause.

Having sought to leave Belgium in order to develop further as a player, the midfielder had an agreement with the club's previous board that they would not put too big a price-tag on him. The fee agreed is thought to be in the region of €8 million.

Current chairman Roland Duchatelet, who recently took over after buying all the club's shares, is not entirely happy with the deal. "These things bother me," he said. "I knew some of the best players would probably leave, but I'm not happy about this. A player can always refer to his contract, but the clubs do not seem to have that certainty."

Witsel told the Portuguese press: "I wanted to move to Benfica and I am happy it has turned out so well. Benfica is one of the great clubs in Europe.

"It was time for me to move after six years in the Belgian league. Portuguese football is very technical and that suits me. I think my performances with Standard in Europe must have pleased Benfica and I also showed them what I could do in the Belgian play-offs."

Witsel won the Belgian league twice with Standard and was awarded Belgium's highest individual accolade, the 'Golden Shoe', in 2008. Witsel was also influential in winning the Belgian Cup last season.

He added: "Winning the Belgian Cup is a great way of saying goodbye to the Standard fans. I played there since I was nine and want to thanks the fans and the club for their relentless support. I hope many Standard fans will keep track of what I do."

Portuguese legend Rui Costa, who is on the board of the club, said of the transfer: "We had been watching Axel for a while and absolutely wanted to get him to Benfica. We are happy he has made the move. We can be very ambitious with him in the team.

"Even at his young age, he has a lot of experience at international level. He is amazingly fit and has a great margin to get even better. Witsel will be in the squad straight away. I don't know whether he will play, though - that is up to the manager."

Witsel has scored five goals in 18 games for Belgium.