She said NPS will not attend shareholders' meetings on Monday, Oct. 27, at which Samsung Heavy Industries (>> Samsung Heavy Industries Co.,Ltd) and Samsung Engineering (>> Samsung Engineering Co Ltd) will seek approval for the first to effectively absorb the second unit.

NPS, the world's fourth-largest pension fund, currently owns 5.9 percent of each of the two Samsung units, the spokeswoman said.

On Sept. 1, Samsung Heavy announced the plan to absorb Samsung Engineering for about $2.5 billion, the latest step in a restructuring at South Korea's largest conglomerate.

Since the merger plan was announced, shares of both units have fallen more than 20 percent. The plan could be derailed if enough shareholders join NPS in asking for stock buybacks or oppose the merger at Monday's meetings.

At the offered buyback prices - more than 15 percent above the current share prices - the combined NPS stakes in the two companies are worth about 520 billion Korean won ($492.2 million).

Under South Korean rules, shareholders who decide not to participate in a merger can exit by asking the merging companies to buy back their stakes.

Since the merger announcement, shares in Samsung Heavy have fallen 21 percent as of Friday, while those in Samsung Engineering have dropped 24.5 percent. Analysts have said the companies' low-margin projects under construction and difficulty of finding short-term synergy between them have contributed to the price drops.

If the merger is approved on Monday, NPS only wants to retain its right to ask for a stock buyback, its spokeswoman said.

It is unclear what other third-party shareholders will decide. As of Friday, if about 18 percent of Samsung Heavy shareholders or about 19 percent of Samsung Engineering shareholders ask for stock buybacks, the merger agreement could be cancelled by one of the companies, according to a Samsung Heavy filing in September.

A merger needs the approval of two-thirds of shareholders attending the meeting and one-third of all shareholders to pass, under South Korean rules. As of June, about 22 percent of Samsung Engineering and 24 percent stake of Samsung Heavy were held by Samsung Group affiliates.

A Samsung Heavy spokesman and a Samsung Engineering spokeswoman could not be reached.

(Reporting by Joyce Lee; Editing by Richard Borsuk)