SEOUL (Reuters) - An influential investor advisor has urged Samsung C&T Corp (>> Samsung C&T) shareholders to reject a merger with a sister firm that would pave the way for a generational leadership change at South Korea's biggest family-run conglomerate.

Ahead of what investors expect to be a tightly contested July 17 vote, advisor firm Glass Lewis & Co said in a note dated Wednesday that Cheil Industries Inc's (>> Cheil Industries Inc) $8 billion all-stock offer was "profoundly unattractive" for C&T investors and "exceedingly advantageous" for Cheil shareholders.

The recommendation bolsters U.S. hedge fund Elliott Associates' position in a rare case of shareholder activism in South Korea. Elliott, which holds 7.1 percent of builder C&T, has mounted legal challenges to the merger and is urging shareholders to vote against it.

"We are pleased to see that independent specialist advisors agree with our position that the proposed merger is without merit," Elliott said in a statement.

While a court in Seoul on Wednesday dismissed Elliott's request for an injunction to stop the shareholder vote, Glass Lewis's report could encourage more institutional investors to reject the offer.

C&T shareholders such as Aberdeen Asset Management, APG Asset Management and Ilsung Pharmaceutical Co (>> ILSUNG PHARMACEUTICAL CO.,LTD) have publicly criticised the deal.

"We believe SCT (Samsung C&T) investors should reject the arrangement and encourage the board to more thoroughly consider alternatives available to the company," the advisory firm said.

Another major proxy advisor, Institutional Shareholder Services Inc, is expected to issue its own recommendations by the end of this week.

Samsung C&T needs at least two-thirds of the votes cast on July 17 to proceed with the deal.

The deal would allow the heirs of Samsung Group [SAGR.UL] patriarch Lee Kun-hee, who remains hospitalised since a May 2014 heart attack, to consolidate stakes in affiliates like smartphone maker Samsung Electronics Co Ltd (>> Samsung Electronics Co Ltd) and keep control of the vast business empire.

A Samsung C&T spokesman said the proposed merger was in accordance with the law and that the company wanted to boost shareholder value.

Cheil, the Samsung Group's de facto holding company, and Samsung C&T have been lobbying shareholders to vote in support of the merger, promising greater post-merger shareholder returns and an improved governance structure to protect investor interests.

Cheil is being advised by Morgan Stanley (>> Morgan Stanley), while Samsung C&T is being advised by Goldman Sachs (>> Goldman Sachs Group Inc) and Credit Suisse (>> Credit Suisse Group AG).

(Additional reporting by Joyce Lee; Editing by Tony Munroe and Stephen Coates)

By Se Young Lee