Norway's sovereign wealth fund, Europe's biggest equity investor, on Wednesday criticized German auto maker Volkswagen for plans to take over some Porsche assets, which it called costly and unacceptable.

The wealth fund, which at the end of 2008 held stock worth $450 million in Volkswagen, did not openly threaten to sell its shares but said it saw little reason to support the proposed transactions if its concerns were not alleviated.

The planned transactions...leave the impression of being designed to suit the needs of the Porsche controlling families at the expense of Volkswagen and its non-controlling owners, the Norwegian fund said in an open letter to VW board members.

Volkswagen was not immediately available for comment.

If its concerns persisted as (an) investor, we will consider the options open to us in this respect, the $420 billion fund said in the letter, published on its website.

The fund said the information about Volkswagen transactions with Porsche entities and controlling owners fails to assure minority owners of the plans' benefit.

In total, therefore, the deal appears unacceptable, said the fund, which invests Norway's revenues from oil and gas activities in offshore stocks and bonds.

The central bank-run fund called on Volkswagen to cancel plans to assist the Porsche and Piech families by buying out their privately held automotive trading business of Porsche Holding Salzburg, unless it demonstrates how the acquisitions have particular strategic value.

It also questioned the terms on which Volkswagen aims at investing in Porsche's car manufacturing division, saying the market seems to judge that Volkswagen currently plans to pay a rich price for those assets.

It asked for an independent appraisal of the assets and debt and questioned the financial logic behind raising capital in Volkswagen's preference share class only, given the more attractive pricing of its ordinary shares.

We call on the supervisory board to prepare for raising capital in the ordinary, voting share class. An approval of this will help demonstrate that ordinary shareholders believe a capital raising enhances value, it said.

Norway's wealth fund is becoming an increasingly active financial shareholder, seeking high corporate governance, environmental and ethical standards for the more than 8,000 companies in which it holds stakes.

(Reporting by Wojciech Moskwa; editing by Carol Bishopric)