The Frenchman, 24, spent just one season at promoted Lorient, who he helped to a surprise seventh-placed finish in Ligue 1.

"Koscielny is a central defender with great ability who performed extremely well last season for Lorient," boss Arsene Wenger told the club website.

"We identified him as a very strong centre half, who has made big progress very quickly.

"He has shown he is mentally strong, he's a fighter and a very strong competitor. Koscielny is a great addition to our squad."

Out-of-contract defensive trio Sol Campbell, William Gallas and Mikael Silvestre have been tipped to leave the North London club, leading Wenger to bring a new centre-half partner for Thomas Vermaelen.

The Gunners signed Morocco striker Marouane Chamakh from Bordeaux earlier in the summer.

"I'm very excited to be joining Arsenal," Koscielny said on the Arsenal website. "They are one of the biggest teams in Europe with many great players.

"I'm really looking forward to playing in the Premier League and the Champions League with my new club. Also, in Arsene Wenger, Arsenal has one of the best managers in the world and I can't wait to work with him and do my best for the Arsenal supporters."

Koscielny had never played in the French top flight before last season, having previously represented Ligue 2 sides Tours and Guingcamp.

He is also eligible to play for Poland due to his ancestry.

PA Sport