Boro chief executive Keith Lamb has dismissed suggestions the club have rejected a £10million offer for the 24-year-old Nigeria international, with Pompey having made no secret of their interest in taking the player back to Fratton Park.
Boro have spent the summer fending off rumours that Yakubu is unhappy at the Riverside Stadium and ready to leave.
However, it is understood that a substantial offer for the African, who has scored 35 goals in his two season on Teesside, could persuade them to re-assess their stance.
Manager Gareth Southgate has repeatedly spoken of his intention to keep his best players, and earlier this week warned Everton off after reports suggested David Moyes had identified England winger Stewart Downing as a £10million target.
But with Yakubu having stalled on the goals front after the turn of the year in each of the last two seasons, and six of the 16 goals he claimed during the last campaign having come from the penalty spot, the club could be prepared to do a deal.
It is understood Pompey have made an enquiry and been informed of Boro's valuation, but a formal offer is yet to be made.