SEOUL, July 11 (Reuters) - South Korea's National Pension Service (NPS), the biggest investor in Samsung C&T Corp , has decided to vote its shares in favour of an $8 billion all-stock takeover offer from sister firm Cheil Industries Inc, the Korea Economic Daily reported, without citing any sources.

A spokesman for the NPS declined to comment to Reuters.

NPS holds an 11.2 voting stake in Samsung C&T, a construction firm, and its vote could be decisive in what is expected to be a close vote at a July 17 shareholder meeting.

The merger, seen as key to a generational leadership transfer at Samsung Group, has drawn opposition from activist U.S. hedge fund Elliott Associates.

NPS decided in an internal meeting on Friday it had decided to support the merger, based on the positive outlook for the combined firm's share price and the group's plans to bolster shareholder rights, such as setting up a governance panel and expanding dividends, the newspaper reported late on Friday.

On Friday, Yonhap news agency reported the NPS had reached a decision, but would not announce its vote until after the July 17 meeting.

(Reporting by Joyce Lee; Writing by Tony Munroe; Editing by Clarence Fernandez)