Armitage left London Irish at the end of last season and joined his brothers, Steffon and Delon, at the French giants penning a three-year-deal.
London Irish is currently bereft of centres with Joe Ansbro out with a long-term injury and Setaimata Sa yet to arrive at the Club.
"I was very keen for Guy to stay on at the club when I arrived back at London Irish so I am delighted he has re-joined us for the season," said London Irish director of rugby Brian Smith.
"Guy's development at Toulon was always going to be in stages and it makes sense in season one to come back for first team game time and continue his development."
Armitage is excited about his return to the Exiles.
"It is great to be back at London Irish and I am looking forward to playing some competitive rugby," he said.
"I cannot wait to get started."
Armitage has played through all England age representative teams after being identified as a talent while with Brixham RFC. He turned out regularly for Brixham up to the age of 15 before earning a scholarship to Wellington College in Berkshire.
Armitage made his London Irish debut, alongside Steffon, in the 39-14 loss to Sale Sharks in the LV= Cup in 2010. Guy has made nine appearances for the Exiles including four in the Aviva Premiership.
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