UPDATE 3-Von Finck family sells chunk of SGS stake

* Von Finck family sells 40 pct of its 25 pct SGS stake

* Stake worth around $990 mln based on Tuesday close

* Sells stake to Goldman Sachs, Deutsche Bank for placing

* SGS declines comment on whether it will buy shares

* Shares down 6 pct

(Adds analyst comment)

By Katie Reid

ZURICH, Nov 11 (Reuters) - SGS (SGSN.VX), which quality checks goods from London's black cabs to toys, said its largest shareholder had almost halved its stake, prompting the inspection services company to reiterate its full-year guidance.

SGS gave no explanation on Wednesday as to why Germany's von Finck family had sold the 10 percent stake, worth around 1 billion Swiss francs ($992.1 million) based on Tuesday's closing share price, and its shares tumbled to a fresh three-month low.

The von Fincks held 25 percent of SGS and have now sold 40 percent of that holding to Goldman Sachs (GS.N) and Deutsche Bank (DBKGn.DE), who will place the stake on the market in an accelerated book-building process to begin immediately.

SGS, founded in 1878 by a Latvian immigrant to inspect grain shipments coming into France, declined to comment on whether it would buy back any of the shares.

At 1628 GMT, SGS were down 6.6 percent at 1,280 Swiss francs, making it one of the biggest losers on the FTSEurofist 300 index .FTEU3. Rivals Bureau Veritas (BVI.PA) and Intertek (ITRK.L) were trading around 0.8 percent lower.

The von Finck family founded Merck, Finck & Co, the private bank behind Munich Re MUVGN.DE, Allianz insurance (ALVG.DE) and the Loewenbraeu breweries.

WARNING SIGN

SGS reiterated its 2009 target for single-digit percentage revenue growth in constant currencies and said it was sticking to the maintenance of its full-year operating margin compared to 2008.