Arsene Wenger has strengthened Arsenal's much-criticised defence by signing Thomas Vermaelen, the Ajax captain.

The 23-year-old Belgium international is a left-footed centre back and joins on what the club described as a "long-term" contract. Arsenal did not disclose the fee but it is thought to be about £10 million.

“I’m just so happy to be joining Arsenal,” Vermaelen said. “This is a great club with a world class manager, high quality players and a fantastic stadium. Everything is set up for Arsenal to be very successful for years to come and I joined this club because I know Arsenal will be challenging for trophies.”

Vermaelen made 143 appearances for Ajax, scoring ten times. He has 21 caps for his country and his leadership qualities should be useful in a youthful Arsenal side that was accused of lacking backbone last season. Wenger conceded near the end of the campaign that defensive vulnerability was a key reason for the club's failure to win honours.

“I have had a good taste of the Champions League during my time with Ajax and this is something I am looking forward to experiencing again with Arsenal,” he said. “I just want to assure the Arsenal fans that I will give everything for this club and I hope we can enjoy winning some trophies together next season.”

Vermaelen's arrival will cast doubt on the futures of Kolo Toure and William Gallas. Toure has attracted interest from Manchester City and endurred a difficult time last season, while Gallas was stripped of the captaincy after publicly criticising team-mates and is presently sidelined with a long-term knee injury.

It remains to be seen whether Wenger, the Arsenal manager, will pursue his interest in Brede Hangeland, the Fulham defender, now that Vermaelen is on board.

“Thomas is a great signing for Arsenal Football Club, and we are all delighted that he is joining us,” Wenger said. “Thomas is a solid performer with good experience of Champions League and Uefa Cup football and is also a regular with the Belgium national team. Thomas is primarily a left-footed centre back, but he's multi-functional and can also play at left back. He is strong technically with a positive winner's attitude and I have no doubt his arrival will strengthen our team. Thomas Vermaelen will be a huge asset to Arsenal."