UPDATE 1-PartyGaming co-founder sells shares for $308 mln

* Sale at 250p/share, 12 percent discount to Monday's close

* Will increase institutional free float

* Move could help possible U.S. re-entry

* Dikshit keeps 9 percent stake

* Shares down 12.6 percent

(Adds placing price, analyst comment, shares)

By Matt Scuffham

LONDON, Oct 20 (Reuters) - PartyGaming Plc's (PRTY.L) billionaire co-founder Anurag Dikshit sold two thirds of his 28 percent stake in the world's biggest online gaming firm, raising 187.5 million pounds ($308 million) but knocking the shares.

Dikshit, who was fined $300 million after pleading guilty last December to an online betting charge in the United States, said in a statement on Tuesday he sold 75 million shares or about 18 percent of the company, at a price of 250 pence per share.