Celtic Warriors skipper Thomas, who still has two years remaining on his contract with the region, has been made a lucrative offer by the French giants who are ready to splash out to bring Wales' most prolific try scorer on board
And after visiting the club last week, Thomas, 29, is now expected to join by the end of the week.
Thomas passed Ieuan Evans' record for Wales tries when he scored in this season's Six Nations win against Italy.
"He still has two years remaining on his contract so the matter is in the hands of the club rather than the player," said Warriors coach Lynn Howells.
"We obviously would want him to stay but only if he gives his full supportto the region."
The transfer is sure to spark fears within Wales that more players might follow with several of the current national team being targeted by Europe's wealthiest club.
Stephen Jones has yet to sign a new contract since being approached by Leicester while Shane Williams is also considering offers from abroad. Agent Peter Underhill, who brokered Wales captain Colin Charvis' move to Tarbes, said: "Wales players are in huge demand following he success of the World Cup and the improvement in the Six Nations."