Wales and Lions fly-half Dan Biggar will join Northampton from Ospreys next summer, the Aviva Premiership club have announced.
Biggar has made 56 appearances for Wales, 203 for the Ospreys and toured with the British and Irish Lions to New Zealand earlier this summer, playing in five of the 10 matches. He will link up with his Wales international colleague George North at Franklin Gardens.
"I'm hugely privileged to sign for a club with the history and tradition of Saints," said Biggar.
"It's extremely exciting to start a new chapter of my career with a club that shares my ambition to win trophies, which is where this club belongs.
"This is an opportunity for me and my family to experience rugby in a different environment and playing in a town and surrounding area which has a massive passion for rugby was a huge attraction.
Saints director of rugby Jim Mallinder said: "We're delighted to have secured the signature of such a highly-rated player.
"Dan has shown his excellence year in, year out, for both the Ospreys and Wales, and he performed really well for the Lions this summer.
"The fact that he has put pen to paper a year in advance shows that Dan has plenty of belief in where we are going as a club and our ambitions for the future."
Biggar was on a national dual contract that is funded 60 per cent by the Welsh Rugby Union and 40 per cent by the region and there is concern in Wales over the loss of another top player.
When the 27-year-old moves to the East Midlands, his immediate Wales prospects will hinge on him securing one of four wildcards available to head coach Warren Gatland next season - under the WRU's senior selection policy for players based with clubs outside Wales.
That figure drops to just two wildcards in 2019-20, which is also a World Cup season.
Scrum-half Rhys Webb is another star name who has been linked with a move away from the Liberty Stadium and the Ospreys and WRU have issued a joint statement underlining their disappointment over Biggar's departure.
It reads: "Dan Biggar has informed his employers of his intention to leave Welsh rugby to join Northampton Saints next season.
"Biggar is under National Dual Contract with the Ospreys and Welsh Rugby Union for the remainder of the current season, but has declined to take up the option to extend this deal under significantly improved terms.
"While this is disappointing, as the intention is to keep our best players in Wales, there is an appreciation that on occasions market forces will make this impractical.
"The high level of Biggar's contribution to Wales and the Ospreys during his career to date, and his continued professionalism in this regard, is unquestioned and all parties continue to wish him well for the future."