The 24-year-old has been linked with a move away from the Riverside Stadium throughout the summer with Everton currently thought to be leading the race for his signature.

PA Sport understands the Nigeria international did not train with his team-mates either today or yesterday, further fuelling talk that his days with the club he joined from Portsmouth for $18.5million in July 2005 could be drawing to a close.

Boro were not commenting on Yakubu's situation, although manager Gareth Southgate has repeatedly insisted he is not for sale.

However, the Teessiders have been actively pursuing potential replacements for much of the last few months knowing that his continued presence at the Riverside is not certain.

Everton's current valuation of the player is understood to be around $2.5million short of the figure at which Boro might be prepared to do business, while former club Pompey and Manchester City have been monitoring the player's situation for some time.