Spanish full-back Porro trained in Lisbon on Sunday ahead of a move to north London which follows weeks of talks between the clubs over his £39million release clause.
Key to the deal has been Sporting’s ability to bring in a replacement defender on their right flank.
Former Arsenal player Bellerin is on the verge of filling that hole in a loan switch from the Camp Nou.
The Catalans signed the 27-year-old on a free transfer last summer after beating Real Betis to his signature, yet he has only played seven times this season with the vast majority of his time spent on the bench.
Spurs signing Porro, 23, will likely see Djed Spence allowed to leave on loan.
Should Tottenham complete the deal, it will be the second of their January transfer window after Arnaut Danjuma’s loan arrival last week.