HITC Sport understands that Boro have been cleared to complete a deal, with a £7 million transfer now set to be pushed through.
Boro had prepared a huge offer for the England midfielder, paying him close to his West Ham wage packet over the course of a five-year deal.
Boro have also agreed a £7million fee for the player, which they were forced to pay as rivals Sunderland had made a counter offer for the 30-year-old.
The delay had been over the fee and a few details at West Ham's end as they made sure they had cover, and with Argentine Manuel Lanzini due in this week to complete his move from Al Jazira – as revealed by HITC earlier on Wednesday - the East London club have now allowed Downing to make the move.
Downing is now due to have a medical on his native Teesside before linking up with Boro at their pre-season camp in Spain.
A source confirmed to HITC Sport: "When Boro first made the move, nobody thought it was possible but the deal is now done, and everyone is happy."
West Ham boss Slaven Bilic had confirmed to Downing when he first took charge at the Boleyn Ground that he was not in his long-term plans at the club.
In further good news for Boro, Espanyol have announced on their official website that a deal to take Uruguayan forward Christian Stuani to the Riverside Stadium is now set for completion after he passed a medical with the Championship outfit.