Kazunori Yamauchi, who blames the delay on the PS3 version of the game, told Eurogamer: "We had been developing the PSP version, but the problem was that the PS3 version took much more time and effort than we had first imagined.
"So that's been delayed - I don't think we'll be able to make the end of this year for the PSP version, but we are working on it."
The Portable PlayStation edition of Sony's acclaimed racer, which has been in development for over four years, has gone through a number of changes and Yamauchi claims the PS3's online features will link up with the PSP game.
"Once we experienced PS3 online and went through all of that, we came to the conclusion that PSP should not be a standalone product - it should be linked in to the world of Gran Turismo, linked with the PlayStation 3," said Yamauchi.
The PSP edition of Gran Turismo was originally announced in 2004 and due for release in spring 2005 in Japan, but delays have seen it put back numerous times.
The Gran Turismo series is one of the world's biggest-selling video game franchises, with 48 million copies sold since its original PlayStation release. The latest, Gran Turismo 5: Prologue for PS3, became the fastest-selling PS3 title during its first week on sale.