UPDATE 4-Starboard to vote against Celgene/Bristol deal, move ahead with proxy

proxy@ (Adds details from Starboard letter, updates shares)

Feb 28 (Reuters) - Activist investor Starboard Value LP plans to vote against Bristol-Myers Squibb Co's proposed $74 billion takeover of biotech Celgene Corp and urged other shareholders to join it in voting down the "poorly conceived and ill-advised" deal.

The New York-based hedge fund said in a letter sent to Bristol-Myers shareholders http://www.starboardvalue.com/wp-content/uploads/StarboardtValuetLPtLetterttotBM Y t S h a r e h o l d e r s t 0 2 . 2 8 . 2 0 1 9 . p d f that the Bristol-Myers board should be open to evaluating all alternatives, including a sale of the company. It also said it will move ahead with its campaign to install its own directors on Bristol's board.

The letter comes only hours after Bristol-Myers' second biggest shareholder, Wellington Management, publicly said it was not supporting the deal. Wellington, which owns nearly 8 percent of Bristol-Myers, said the buyout was too risky and expensive.

The shareholder opposition imperils what would be the largest pharmaceutical acquisition of all time.

Starboard criticized the record of Bristol-Myers management, saying the U.S. drugmaker has underperformed the S&P 500 index by more than 40 percent over the tenure of Chief Executive Giovanni Caforio.

"These results are not reflective of a management team and Board of Directors that has earned the right, in our view, to execute on a 'bet the company' acquisition," the letter from Starboard Chief Investment Officer Jeffrey Smith said.

Bristol-Myers investors are due to vote on the deal at a special meeting scheduled for April 12.

Celgene's shares fell more than 7 percent on Thursday while Bristol-Myers was up nearly 1 percent.

Starboard, one of the industry's most prominent and successful activist investors, also said that it will sue Bristol for documents in Delaware court to gain a better understand how management decided to make the offer for Celgene.

One of Starboard's biggest criticisms of the deal is that Celgene's blockbuster drugs will soon lose patent protection and that its pipeline is "risky and will continue to require significant research and development."

Reuters reported earlier this month that Starboard was working with a proxy solicitor to gauge the level of support among Bristol-Myers shareholders for the Celgene deal.

Bristol-Myers said Starboard informed the company it had bought roughly one million of its common shares. That is equivalent to a roughly $50 million stake, a sliver of the company's $84 billion market value.

Bristol has told employees that it is disappointed by Wellington's opposition to the Celgene deal, and said it would press ahead.

"Even with the combined voting power for both Wellington and Starboard Value, we believe there continues to be a high hurdle for opposition to reach majority," said Andy Hsieh, a William Blair analyst.

Sources have told Reuters that Dodge & Cox, Bristol-Myers' fifth largest shareholder, is also unhappy with the deal.

Bristol and Celgene through the deal hope to create a market leader in the lucrative treatment of cancer through their combined portfolios.

(Reporting by Svea Herbst-Bayliss and Michael Erman in New York; Additional reporting by Ankur Banerjee and Manas Mishra in Bengaluru Editing by Shounak Dasgupta and Bill Berkrot)