The two clubs had agreed a fee in the region of £20m for the 25-year-old, who was believed to be keen on a move to White Hart Lane.
However, a third-party investor currently owns 15% of Moutinho's economic rights and would therefore have been entitled to a cut of the transfer fee.
Because third-party ownership is forbidden under Premier League rules, Spurs required the investor to be bought out by the Portuguese club.
According to The Guardian, this revelation led to Porto going back on the previously-agreed deal and demanding a higher fee from Spurs for the player.
Moutinho's current contract at Estadio do Dragao expires in June 2015.