Aitor Karanka's future as Middlesbrough manager is shrouded in doubt after the Championship promotion contenders announced he will not be in charge for Sunday's trip to Charlton.

Karanka is believed to be unhappy with his players after some poor performances in recent weeks and the Spaniard stormed away from Middlesbrough's training ground on Friday following a heated meeting with the squad.

The 42-year-old questioned the players' desire to win promotion to the Premier League following their midweek loss at lowly Rotherham and is said to have told them he no longer wanted to manage them during the training ground showdown talks.

Karanka's dramatic departure triggered speculation that he is set to quit and that outcome remains likely after Middlesbrough confirmed assistant coach Steve Agnew will take charge at Charlton on Sunday.

"Middlesbrough Football Club can confirm that Aitor Karanka did not take training on Saturday morning," the statement read.

"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."

After losing to Norwich in last year's play-off final, Karanka, who worked as Jose Mourinho's assistant at Real Madrid before taking his first managerial role with Middlesbrough in 2013, was under pressure to get the north-east based outfit into the top-flight this term.

They are second in the Championship table, four points behind leaders Burnley with a game in hand and two points clear of the chasing pack.

But 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 in the race to reach the Premier League.