Billionaire investor Kirk Kerkorian makes official in a regulatory filing today an intention to increase his stake in Ford Motor Co. beyond 5.6%, but the auto maker warns investors to hold their shares until the company’s board can review the offer.

The announcement by Ford reverses a sentiment the auto maker expressed yesterday, when it called Kerkorian’s interest a welcome sign that its turnaround was progressing.

Kerkorian began buying Ford shares April 2 and currently holds 4.7% of the auto maker’s outstanding stock. But he says in a Securities and Exchange Commission filing today his Tracinda Corp. investment group will pay $8.50 for an additional 20 million shares that will give him the 5.6% stake.

Investors must file such notices with SEC if a stock purchase pushes their investment beyond a certain threshold.

Ford says it will advise stockholders of the board’s position by May 22, as required by securities laws. However, investors are free to sell their shares at any time.

Kerkorian’s offer represents an estimated 13.3% premium over Ford’s closing stock price of $7.50 on April 25, which was the last trading day before Tracinda announced its intention to purchase additional stock.

In the filing, Kerkorian calls Ford “an attractive investment,” but the document also reveals how a major investor like Kerkorian maneuvers with high-ranking executives to gain such a massive stake.

For example, Kerkorian adviser and former Chrysler Corp. executive Jerry York met on April 4 with Ford CEO Alan Mulally and CFO Donald Leclair, according to the filing. York informed the executives during the meeting of Tracinda’s interest in investing in Ford. Three weeks later, York told Leclair of Tracinda’s purchase of 100 million shares of Ford stock and its intention to buy another 20 million shares.

About a week later, York spoke again with Leclair to apologize for “off-the-cuff” remarks he made to the automotive press concerning Ford’s intention to sell its Volvo and Mercury brands, a claim Ford subsequently denied. York also took the opportunity to “reiterate his and Tracinda’s belief in Ford management and admiration for their accomplishments.”

Six days after that conversation, Tracinda representatives told Ford it would commence its offer on May 9.

Concurrent to the SEC filing, Kerkorian has placed a solicitation for additional shares in The Wall Street Journal. The solicitation expires June 9.

The move is the latest attempt by Kerkorian to take a large stake in a U.S.-based auto maker.

Last year, Tracinda made an unsuccessful $4.5 billion bid to acquire the former Chrysler Group from DaimlerChrysler AG. In 2006, the investment firm dumped its 10% stake in General Motors Corp. after attempts to create a 3-way hookup with Nissan Motor Co. Ltd. and Renault SA failed.

In a recent statement, Tracinda says its interest in Ford is due to the auto maker’s performance during difficult economic conditions. “Tracinda believes that Ford management under the leadership of CEO Alan Mulally will continue to show significant improvements in its results going forward.”