However the Exiles have come to an agreement with Armitage that paves the way for him to return to France, where he grew up as a child.
And coach Philippe Saint-Andre was quick to welcome his new arrival who will replace Australian George Smith.
He said: "He's a very fast player and very powerful, given his size and his weight. He can also play at centre and he speaks French."
The move, however, will almost certainly stop him adding to his five England caps - the last of which he won against Italy in the 2010 RBS 6 Nations - with the RFU promising to only pick foreign-based players in 'exceptional circumstances'.
The openside flanker had already slipped down the pecking order for the No.7 shirt with James Haskell starting there in the 2011 RBS 6 Nations while both captain Lewis Moody and Hendre Fourie are also ahead of him.
Ironically, Armitage's replacement at Irish could be Fourie after Leeds accused the Exiles of making an illegal approach for his services.