COPENHAGEN, Dec 3 (Reuters) - The main shareholders in Denmark's ISS said late on Tuesday they launched an accelerated bookbuilt offering to institutional investors of up to 25 million existing shares.

The shareholders, private equity firm EQT and funds advised by Goldman Sachs, initially planned to sell 18.6 million shares in a book-building process, but decided later on Tuesday to increase this to 25 million shares.

Shares in ISS closed at 169.20 Danish crowns per share on Tuesday, and assuming the 25 million shares are sold at that price, the deal is valued at 4.2 billion Danish crowns ($699 million).

EQT and the Goldman Sachs-advised funds' ownership will be reduced to 19.3 percent after the sale from 32.7 percent.

($1 = 6.0094 Danish crown) (Reporting by Ole Mikkelsen; Editing by Anupama Dwivedi)