Porsche is set to develop its eagerly awaited fourth model line - already named the Panamera - by itself rather than seek a partner, industry sources have told Reuters.

The news agency noted that would mark a departure from its strategy with the Cayenne sport utility vehicle which was developed with Volkswagen and which VW part-makes in Slovakia before handing over to Porsche for final assembly in Leipzig, eastern Germany.

Porsche reportedly said it would wait until the Frankfurt motor show in September to reveal its hand for the fourth line, but industry sources told Reuters there was little doubt it would proceed with the plan it has kicked around for more than three years.

"The fourth model line is coming," one executive reportedly said. "Porsche would lose face if the fourth model line does not come on to the market."

Reuters said demand has been such that production of the Cayenne is near full annual capacity of 41,000 units - around half of the more than 80,000 units Porsche aims to sell in its current fiscal year to July.

"We have to invent a vehicle like the Cayenne again," one Porsche manager told the news agency, although he would not comment on market expectations that the new car would have four doors.

People familiar with the project have told Reuters profitable Porsche intends to develop and build the car on its own, perhaps based on building blocks provided by suppliers such as ThyssenKrupp Automotive, Boxster manufacturer Valmet or Karmann.

The sources reportedly dismissed media reports that Porsche would cooperate with vehicle development and assembly specialist Magna Steyr in Austria on the project.

The Porsche plant in Leipzig has plenty of room to expand output to handle the new car, Reuters said.