Borthwick turned down Sarries deal
Steve Borthwick turned down one more year at Saracens to take up the "huge attraction" of coaching Japan alongside Eddie Jones.
Saracens captain Borthwick will end 16 years of top-flight rugby in Saturday's Aviva Premiership final against Northampton Saints at Twickenham.
The former England captain will assume a permanent role in Japan's backroom staff this summer, after two years of part-time work with the Cherry Blossoms.
Former Australia coach Jones was responsible for taking Borthwick from Bath to Saracens in 2008, and the focused lock is already relishing renewing old ties.
"The big opportunity for me is to work under a great coach like Eddie Jones," said Borthwick.
"I think that's particularly important for me in my development to work under such an experienced coach.
"That's a huge attraction.
"For the last couple of years Eddie gave me an opportunity to be involved with Japan, and that relationship has grown.
"Eddie asked me to join the team on a full-time basis once I retired from playing, and it's an opportunity to go and work under a fantastic coach like Eddie Jones, and learn, which has already been great for my development.
"And will continue to be great for my development."
Shoulder trouble almost robbed Borthwick of his chance to end his career with back-to-back finals, the gritty second-row trudging out of Saracens' 31-17 Premiership semi-final victory over Harlequins at Allianz Park on May 17.
The north Londoners' line-out coordinator forced his way back for last weekend's 23-6 Heineken Cup final defeat to Toulon in Cardiff though, and will now face Saints at Twickenham on Saturday.
Borthwick admitted he weighed up committing to one more year in the ranks at Saracens, but eventually opted to launch a full-time coaching career instead.
"There was the opportunity to continue playing, and Saracens gave me lots of time to consider that, and I wanted to do right by Saracens," he said.
"I wanted to make the right decision for myself, but I wanted to do right by Saracens, give them lots of time to plan ahead.
"It would have been pretty poor of me if I'd waited until the end of the season and then said 'Mark, thanks I don't want to play any more'.
"That would have left them in a poor situation for next year.
"So I was very conscious I wanted to tell Mark very early on whether I would carry on playing or not.
"I needed to make that decision regardless of coaching opportunities.
"I think the clean break is an advantage for me to go somewhere different, and to have the opportunity to stay involved in rugby is a huge privilege."