Shares in Taiwanese smartphone maker HTC tumbled as much as 6.5 percent on Monday after losing a preliminary ruling in a patent case with Apple Inc, with its announcement of a share buyback failing to stem the fall.

On Friday Apple won a preliminary ruling from a U.S. trade panel that HTC infringed on two of the California company's patents, the first step in an acrimonious dispute between the two in the cut-throat smartphone market.

HTC, with an eye on the damage to its shares from the ruling, said over the weekend it would buy back up to 20 million of its own shares before Sept 17, or about 2.4 percent of its outstanding shares.

The buyback price will be between T$900 ($31) and T$1,100 per share. The first 10 million will be bought back from July 18 and the second from August 18. The company will give half the bought-back shares to staff and retire the rest.

HTC was intending to tell investors T$900 would be the shares' bottom, but obviously investors did not buy that, said Simon Liu, deputy investment officer of Polaris Financial Group's fund arm.

Instead, they are betting the ITC ruling would hurt its future earnings and market share. Even worse, more similar legal battles would follow.

At 9:36 ET, HTC shares stood at T$867.0, down 4.4 percent in a broader market down 0.84 pct.

HTC's shares had fallen over 20 percent between July 6 and July 14 on a range of concerns including the lawsuit and speculation that HTC could face a huge compensation payout to Apple, as well as increasing competition in the market place from Samsung and slowing global demand.

HTC's plan to acquire S3 Graphics for $300 million from Via Technologies, which is owned by HTC Chairwoman Cher Wang, also raised concerns over the company's corporate governance.

In the lawsuit, Apple initially accused HTC of infringing 10 patents but six were dropped from the case for various reasons. The ITC judge ruled that HTC infringed two of the remaining four.

A final determination in the case is due on December 6.

Apple has filed a parallel lawsuit against HTC in a court in the U.S. District Court in Delaware.

We are confident we have a strong case for the ITC appeals process and are fully prepared to defend ourselves using all means possible, HTC said in a statement.

HTC will hold an investor conference at the end of this month, when it will give guidance for the third quarter.

(Reporting by Jonathan Standing)

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