Series creator Kazunori Yamauchi has told Eurogamer that Gran Turismo 4 on PSP is unlikely to be out in 2008.

He explained that unforeseen development issues on PS3 projects had taken precedence and forced it out of this calendar year.

"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," Yamauchi revealed.

"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."

Yamauchi went on to explain that the PSP version had changed considerably since it was announced back in 2004, and now looked likely to include online features inspired by the PS3 Gran Turismo games.

"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 Gran Turismo series has shipped over 48 million units around the world since it was first released on the original PlayStation.

Gran Turismo 5 Prologue came out last Friday and raced straight to the top of the charts, becoming the fastest-selling PS3 title over the course of a week.

It's good, too, as our Gran Turismo 5 Prologue review will tell you.

Look out for the full interview with Kazunori Yamauchi this afternoon.