Aitor Karanka's future as Middlesbrough head coach is increasingly uncertain after club officials confirmed Steve Agnew will take charge at Charlton Athletic.
The Spaniard is understood to have left the training ground unexpectedly on Friday following a meeting, and his dramatic departure triggered a series of rumours about his future.
The club has since confirmed Karanka did not take training on Saturday morning, with assistant head coach Agnew set to take charge at Charlton tomorrow.
Boro, who travel to Charlton in the Sky Bet Championship on Sunday, went into the weekend sitting in second place in the table, four points behind leaders Burnley with a game in hand and two points clear of the chasing pack.
However, they have won just three of their last 10 games and taken 12 points from a possible 30 to surrender what was once a commanding lead.
A Middlesbrough statement read: "Middlesbrough Football Club can confirm that Aitor Karanka did not take training on Saturday morning.
"Assistant Head Coach Steve Agnew will take charge of the team for Sunday’s fixture at Charlton Athletic.
"We appreciate supporters will want to know further details, but at this time the club cannot make any further comment and is solely focused on Sunday's game at The Valley."
There is also an ongoing debate over Karanka's use of £9million January signing Jordan Rhodes, who has started just four times in eight appearances and scored a single goal since his arrival from Blackburn.