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.

A source close to the operation said Goldman has always said it would sell as a private equity investor when market conditions were right but could not confirm the timing.

"The sale operation will take place via an accelerated bookbuilding aimed at institutional investors in Italy and abroad," Goldman Sachs said. Prysmian is 30.3 percent owned by Goldman's Luxembourg unit Prysmian (Lux) II Sarl and 1.47 percent by Goldman Sachs International.

One broker said clients had said the shares were being offered at around 12 euros each while a second broker said the price range was "11.80 euros to 12 euros something". Both brokers said they expect the operation to close Monday evening.

One broker said Prysmian was trading on cheap multiples and Goldman could get more for its stake from a trade buyer.

The Goldman investors in Prysmian will agree to a 60 days lock-up on further stake sales once the price is set, Goldman Sachs said.

(Writing by Nigel Tutt; Editing by Dan Lalor)

((nigel.tutt@thomsonreuters.com; +39 02 66129723; Reuters Messaging:nigel.tutt.reuters.com@reuters.net)) Keywords: PRYSMIAN/

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