UPDATE 1-Goldman Sachs selling 12 percent of Prysmian

* Goldman Sachs has 31.8 percent stake in Prysmian

* Plans to sell to institutional investors

* Shares down 4 percent in after hours trading

(Adds detail, shares)

MILAN, Nov 9 (Reuters) - U.S. bank Goldman Sachs (GS.N) is to sell nearly a third of its 31.8 percent stake in Italian cable maker Prysmian (PRY.MI) to institutional investors, in a process brokers said should close on Monday.

Goldman Sachs is selling 12 percent in Prysmian in an accelerated bookbuilding operation which the U.S. bank said was launched on Monday.

Brokers said Goldman was offering the shares in a range from 11.8 euros per share to above 12 euros.

Prysmian closed down 0.6 percent at 12.90 euros and fell another 3.9 percent to 12.20 euros in after-hours trading.