London Irish bosses are "disappointed" England wing Marland Yarde has chosen to join Harlequins on a two-year contract in the summer.
Irish chiefs had used their recent takeover and influx of funds and board stability to produce a competitive new deal for the 21-year-old.
Exiles academy graduate Yarde has rejected those terms, though, and completed the formalities on his impending Harlequins switch.
Irish said in a statement: "We did everything within our power to retain Marland, considering that the club had developed him as a player through our junior and senior academy programme, progressing to being a valued member of the senior playing squad and ultimate achievement of international honours.
"An offer of more than substantial new terms was made to Marland so, in the circumstances, the club is disappointed with his decision to move on.
"However, the club must also move on and proceed with our plans to build a team for the future."
Irish lost a clutch of top young talent to Bath in the summer - Jonathan Joseph, Anthony Watson, Matt Garvey and David Sisi following coaches Toby Booth and Neal Hatley to the Recreation Ground.
Rugby director Brian Smith had vowed Irish would never let history repeat itself, but the Sunbury-based club have been unable to retain their chief asset.
Former England attack coach Smith will be most upset with Yarde's decision, given the strike-running wing has already forced his way into Stuart Lancaster's full international squad.
Yarde has been ruled out of RBS 6 Nations action with hip trouble, but is an integral part of Lancaster's plans for the future.
Admitting it was a tough call to decide to leave Irish, Yarde said: "I will always have fond memories of London Irish and can only thank the club for what they have done for me as a player to date and the opportunity they have given me.
"I am excited to move on to the next stage of my career with Harlequins and hope to continue to develop and improve as a player."
Harlequins boss Conor O'Shea believes the club have secured the services of a wing with true "world-class potential".
"Having played all his rugby at London Irish and in speaking to Marland and his mother these past few weeks, I know how much Marland appreciates everything that has been done for him by London Irish," said O'Shea.
"However, once he decided that he wanted to make a change I think we both knew the fit was perfect.
"He is young, ambitious, has world-class potential and his 'X-Factor' is with ball in hand. We are absolutely determined that we will help him realise his ambitions and look forward to him joining the squad for pre-season training in the summer."