Harlequins have appointed Leeds assistant coach Brian McDermott to replace head coach Tony Rea.
Australian Rea has been at the club 12 years in various roles
McDermott, a former Bradford player, has signed a two-and-a-half year deal and takes over with immediate effect.
Rea arrived at the old London Broncos as a player in 1994, becoming chief executive officer in 1996 and coach in 2000 after spells as caretaker coach.
The Australian has been appointed to the board of directors and will be in charge of recruitment and the academy.
Rea led the Broncos, who merged with Harlequins rugby union club earlier this year, to the Super League play-offs in 2003 and 2005.
His last game in charge on Saturday saw Quins end a three-game losing run with a 24-16 victory over Castleford.
Rea said the appointment of a British coach would help the club to shed its reputation as an Australian enclave in Super League, and backed McDermott to improve the team's fortunes on the field.
This is not a knee-jerk reaction
"Brian will stamp his own authority on the job," said Rea. "I'm confident of success and I'm really excited to get a British coach.
"I've been so impressed by his technical skills and nous I wanted him to take over immediately."
Quins owner Ian Lenagan said: "Tony and I have been planning this re-structuring for some time.
"It had originally been intended to happen at the start of next season. This is not a knee-jerk reaction to our league position but is rather about putting into place the vital building blocks for a successful future for Harlequins.
"Brian's the best young coach in Super League and is a great acquisition. We believe he is fully ready to make the step up to become a head coach."
McDermott, a former Royal Marine and Great Britain prop, spent 10 years as a player with Bradford, winning the Challenge Cup and three Grand Finals, before starting his coaching career as an assistant at Huddersfield.
He moved with head coach Tony Smith to the Rhinos in 2004, taking temporary charge of the team during Smith's absence through illness earlier this season.
Smith said: "Brian is certainly ready for a head coach's job and goes with all our best wishes and thanks.
This has been the biggest decision of my life but Harlequins is a tremendous opportunity
"There was no way we would have stood in his way once this opportunity became available. He is a very talented coach and I am looking forward to locking horns with him in the future."
McDermott described his time with Leeds as "a dream come true", and thanked Smith for giving him the opportunity to break into the coaching ranks.
"The club have been very professional and treated me better than I could ever have imagined, especially having spent my entire career at their biggest rivals," he said.
"I owe a massive debt of thanks to everyone at the club, especially everyone at our training ground and our chief executive Gary Hetherington.
"In the past, I have never understood why people say decisions like this are the hardest thing they have ever had to do but now I fully understand that.
"This has been the biggest decision of my life and I would never have left Leeds to join just any other club. Harlequins is a tremendous opportunity and a special club."