Javier Mascherano, the Liverpool midfielder, has completed his protracted move to Anfield by signing a four-year contract. The Argentina international has been a key figure at the club since he began his long-term loan deal in January 2007.

The 23-year-old admits he is delighted to have finally resolved his future. "I am very, very happy and now I can concentrate just on playing football," Mascherano said.

"I said all along that I wanted to stay at Liverpool and now that I have signed I can say this is the best and the biggest moment in my career so far. From the first day I arrived at Liverpool the feeling was good and I knew this was where I wanted to play my football.

"It's good news for me and my family that everything is sorted and I know where my future is going to be. I just want to do my best for the fans and the people of the club. I am at a top side and I know I can win titles here. That's the big thing for me. I want to be at a club where we can win trophies and have success."

Rafael Benitez, the Liverpool manager, was also relieved to secure the permanent signing of a "key" player. "We must be really pleased with the situation now," Benitez said.

"He has been a key player for us since he has been here and now we know he can be a key player in our future. From the start we knew Javier was a player with quality and a fantastic mentality. We knew the player when he was in the reserves at River Plate but was playing for the senior national team and we had been monitoring his career since then.

"He is young but he has great experience having played for his country. We knew he would be a good signing for us. It's clear that he is one of the best midfielders in the world but he can get better with us. We now have a lot of good options in the centre of midfield and that's good news for the club."

Rick Parry, the Liverpool chief executive, worked painstakingly to finalise the deal, and said: "Our overriding thought is that we are delighted to have signed a world-class player on a long contract and I know Javier is thrilled too because he was keen to have his long-term future resolved.

"I think this signing demonstrates two things. Firstly, that life at the club goes on despite all of the speculation to the contrary. And secondly it demonstrates in very large measure the ongoing commitment of the owners who once again have come good when funds have been needed."

Mascherano was able to sign for Liverpool outside the transfer window because his contract was owned by the Media Sports Investments consortium.