UPDATE 1-Saab says likely to unveil new owner this week

* GM unit says announcement could come any time

* Says creditors back debt writedown ahead of court hearing

(Adds report of Koenigsegg filing to register new firm)

STOCKHOLM, June 15 (Reuters) - Carmaker Saab said on Monday it was likely to unveil its new owner this week and over three quarters of its creditors had agreed to a proposed writedown of the loss-making General Motors GMGMQ.PK brand's debt.

A source told Reuters last week luxury carmaker Koenigsegg and Norwegian investors had reached a preliminary agreement to buy Saab Automobile, which was put up for sale by its bankrupt U.S parent earlier this year. [ID:nLB799437]

A Saab spokeswoman said an announcement should come soon.

"I think that during this week, chances are good. It could be any time now," spokeswoman Gunilla Gustavs said.

Saab, which first sought protection from creditors in February and was granted an extension until Aug. 20 to get time to restructure and line up a new owner, said more than 80 percent of its creditors had agreed to write down the value of their Saab debt.

The Swedish carmaker has said it needs $1 billion in financing to see it through the crisis and has asked creditors to write off 75 percent of the company's 10.6 billion crown ($1.4 billion) debt, most of which is owed to GM.