Ireland back row forward Sean O'Brien will join London Irish from Leinster after this year's World Cup in Japan, it was announced on Monday.
The 31-year-old, a veteran of 54 Tests for Ireland and two British and Irish Lions tours, will move to the Exiles in December following more than 10 years with Dublin-based province.
By then, London Irish will hope to be back playing in England's elite Premiership -- they sit top of the second-tier Championship more than half way through the campaign.
O'Brien has won four European Cup titles and two Pro 14 finals with Leinster since making the first of his 122 appearances for the province in 2008.
"This is one of the hardest decisions I have ever had to make. I have played all my rugby in Ireland," O'Brien told Leinster's website.
"That being said, I feel the time has come to explore new opportunities, to challenge myself at a new environment, in a new city, with a new club."
And in a separate interview with the London Irish site, he added: "I'm keen to roll up the sleeves and use my experience to help re-establish London Irish as a top club in England."
London Irish's director of rugby is former Ireland coach Declan Kidney, who gave O'Brien his international debut in 2009.
"Sean has proven himself time and time again to be a quality player who sets very high standards for himself and those around him, he leads by example and has a winning mentality," said Kidney.