The Chronicle reports that Newcastle are considering keeping Longstaff as part of the squad for the Premier League campaign.
Pompey were hoping to get the player, but it appears the 20-year-old has been convinced to remain at Newcastle for now.
What is more likely is the player will see how he gets on during the first half of the season.
If he is not part of the first team picture by January, he will go out on loan to a club that offers him more game time.
Longstaff impressed manager Rafa Benitez on the preseason tour, which is a good sign for his chances of playing in competitive games.
Speaking about Longstaff recently, Benitez is quoted by the Chronicle Live as saying: "I think he's done well. He did well in the first game and physically he's fine.
"In this case, Sean has a good condition and he as to play 90 minutes and he played really well."
"We want to give young players an opportunity."