The 21-year-old previously departed the Exiles to join the French outfit last summer, but has since returned to the Madejski Stadium.
"Guy Armitage is fast becoming one of the most dynamic power centres in the country. He's a genuine Premiership player with the ability to go on and play for England like his two older brothers," London Irish director of rugby Brian Smith told the club's official website.
"Guy has forced his way into the starting team this season and if he keeps performing may well be considered for England's summer tour of Argentina."
Armitage has been a regular starter in his second spell with Irish.