UPDATE 2-China Mengniu Dairy to sell $800 mln stake - source

* Private equity fund Hopu and COFCO seen investing in Mengniu - source

* Mengniu shares suspended pending an announcement

* Investment seen as bet on Mengniu's long-term prospects

(Add details and background)

By Michael Flaherty and Donny Kwok

HONG KONG, July 6 (Reuters) - Two Chinese entities are set to invest around $800 million for 20 percent of China Mengniu Dairy (2319.HK), a source familiar with the matter said, the latest cash infusion for an industry recovering from a tainted milk scandal.

One investor would be Hopu Investment Management, a private equity fund founded by Fang Fenglei, chairman of Goldman Sachs' (GS.N) China joint venture, a person familiar with the matter said on Monday.

Mengniu was likely to make an official announcement later in the day, he added, speaking on condition his name not be used because he was not authorised to talk to the media.

The other investor would be China National Oils, Foodstuffs and Cereals Corp (COFCO), the country's largest food importer and exporter, the Wall Street Journal reported.