ZURICH (Reuters) - U.S. shareholder adviser ISS has recommended Holcim (>> Holcim Ltd) investors back the Swiss cement maker's proposed merger with France's Lafarge (>> LAFARGE) ahead of next month's shareholder vote on the $40 billion (26.78 billion) tie-up.

Backing from ISS is a boost to Zurich-based Holcim as it seeks to win the support of two-thirds of investors at a shareholder meeting on May 8 in order to approve a fundraising to pay for the merger.

ISS said in its report to investors that the deal warranted investors' support as it had a clear strategic rationale and offered upside to Holcim investors through synergies.

However, the shareholder adviser stressed that a successful integration of the two companies under the combination's proposed CEO was key.

"The companies are betting that American Eric Olsen will bridge the cultural gap," ISS said in the report, dated April 20.

"But Olsen is untested in the role at either of these firms, let alone at the task of integrating them into the world's largest cement company. It may well be that execution on the operational and strategic fronts is slowed until the cultural issues are resolved."

The CEO role was a major sticking point when the tie-up came close to collapse in March, with the two companies eventually picking Lafarge insider Eric Olsen as chief executive earlier this month.

Holcim has gone on the road to persuade its investors to back the deal. The Swiss firm's second-largest one, Eurocement, has given little indication of its plans, while the third-largest -- U.S.-based fund manager Harris Associates -- has said it is leaning towards supporting it.

(Reporting by Joshua Franklin and Oliver Hirt; Editing by Mark Potter)

Stocks treated in this article : LAFARGE, Holcim Ltd