Athletic Bilbao have parted company with coach Marcelo Bielsa after two years in charge.
Athletic president Josu Urrutia announced at a press conference on Friday night they had opted not to offer the 57-year-old former Argentina boss a contract for next season, saying the board had unanimously taken the decision in the "best interests" of the club.
The Basque outfit enjoyed a memorable first season under Bielsa but the recently-completed campaign was hugely disappointing as the club found themselves fighting to avoid the drop.
Athletic, one of only three clubs never to have been out of the top flight, finished the season in 12th spot, nine points above the drop zone.
A statement on the club's website confirmed: "Josu Urrutia has announced the decision of the club not to offer a contract renewal to Marcelo Bielsa to lead the club next season."
Urrutia, in quotes reported by the Spanish media, explained: "We needed to have guaranteed our (Primera Division) safety to make decisions and from there we started to evaluate many issues about the sporting and institutional areas.
"Last night, in our latest board meeting, we decided unanimously not to propose the continuity of Bielsa, because it is best for the interests of Athletic and we've opted for a change."
Slade, his assistant Kevin Nugent and goalkeeper coach Kevin Dearden have agreed two-year extensions to their deals, which were due to expire at the end of the coming season.
"I'm delighted to be extending my contract with the club," Slade told their official website. "I feel we've made huge progress on and off the field in my time here and we have some positive foundations in place to build on for next season.
"With the improvements that have gone into the backroom team and the training facilities, there's a chance for Orient to push on now and challenge for a place in the Championship and it would be a magnificent story if we could achieve that."
The 52-year-old joined Orient in 2010 and has led them to two seventh-placed finishes in League One, the club's highest finish for a generation.
