TOKYO (Reuters) - Western Digital Corp WDC.O said on Tuesday it will seek an injunction to block the sale of Toshiba Corp's 6502.T prized semiconductor business to a rival group, upping the ante in an acrimonious battle with its chip venture partner.

The latest legal action by the U.S. firm, which jointly invests in Toshiba's main chip plant, comes in the wake of the Japanese conglomerate's decision last week to sell the unit to a consortium led by Bain Capital LP and South Korean chipmaker SK Hynix 000660.KS.

The $18 billion agreement with the Bain group is, however, still unsigned, with Toshiba telling its main banks this week that Apple Inc AAPL.O, a member of the consortium and an important client, had yet to agree to key terms.

Western Digital’s injunction is being sought with the International Court of Arbitration, where the California-based company, which argues no deal can be done without its consent, initiated proceedings against its partner earlier this year.

A panel of three arbitrators may be formed as early as this week and a decision on the injunction could come late this year before any deal closes, a source familiar with the matter said, declining to be identified due to the sensitivity of the matter. A final ruling on the dispute is not expected before 2019.

The contentious auction has underscored how high the stakes are, as rival suitors, the Japanese government and Toshiba’s creditor banks all squabble over the world’s second biggest producer of NAND memory chips.

For Toshiba, a signed deal would come not a moment too soon as it needs to raise billions of dollars to cover liabilities arising from its now bankrupt U.S. nuclear unit Westinghouse before the end of the financial year in March. If it fails to do that, it could be delisted.

Even if Toshiba manages to sign the deal with the Bain group imminently, it is still cutting it fine as regulatory reviews usually take at least six months.

STAKES HIGH

Western Digital said in a statement that Toshiba's decision had been disappointing, given that it had made major concessions. These included giving up its participation in the consortium it was part of, leaving KKR & Co KKR.N and a state-backed fund, the Innovation Network of Japan (INCJ), as the main investors. It also gave up on a plan to take a future equity stake.

It said it was vehemently opposed to a Bain deal, arguing that the inclusion of SK Hynix, a rival chipmaker, heightens the risk of technology leaks and introduces the risk that the deal may not clear regulatory reviews, unlike the KKR/INCJ bid which does not include a chipmaker.

Toshiba declined to comment.

Western Digital, one of world’s leading makers of hard disc drives, paid some $16 billion last year to acquire SanDisk, Toshiba’s chip joint venture partner since 2000. It sees chips as a key pillar of growth and is desperate to keep the business out of the hands of rival chipmakers.

Just last week, Western Digital filed a fresh arbitration request seeking to stop Toshiba from investing in a new chip facility in Yokkaichi, Japan, unless SanDisk was also allowed to invest.

Toshiba said in August it decided to invest in the new line without Western Digital as they “failed to reach agreement” on joint investment.

Western Digital previously sought an injunction from a California state court to block any sale of the chip unit without its consent. The court ordered Toshiba in July to give Western Digital two weeks’ notice before any deal is closed.