Rhys Webb looks set to join Toulon after Ospreys confirmed the Wales scrum-half will leave the Pro14 club at the end of the season.
Reports emerged on Wednesday that the British and Irish Lion had agreed a switch to France.
Ospreys on Thursday revealed that the 28-year-old had decided to leave his homeland and join "an undisclosed French club".
Webb's departure will serve as a blow not only to Ospreys, but also for Wales head coach Warren Gatland, who can only select four overseas-based players – with the likes of Dan Biggar, Liam Williams, Taulupe Faletau, George North, Jamie Roberts and Owen Williams also set to be among them next year.
Ospreys and the Welsh Rugby Union said in a statement: "The Welsh Rugby Union and the Ospreys can confirm that Rhys Webb has informed them of his intention to leave Welsh rugby next season.
"Webb, who is on a National Dual Contract, will join an undisclosed French club for the 2018-19 season.
"In line with our intention to keep our best players in Wales, every effort to retain the player has been made. Whilst the news is disappointing there is an understanding that on occasion market forces will make this impractical."
Webb said: "It's a huge wrench to be leaving the Ospreys after spending all of my playing career with the team. I have made lifelong friends on and off the pitch and I cannot speak highly enough of the quality of the coaching team, players and all of the staff involved within the Ospreys organisation.
"I will continue to give the region everything I can to make sure we have the best season possible and I would like to conclude by saying a heartfelt thanks to everyone within the Region for helping me achieve my goals within rugby.
"I would like to thank the WRU and the Ospreys for their efforts during this process."