UPDATE 1-Greek SEC watchdog bans short-selling in stocks

* Ban follows investor sell-off on Tuesday

* Rule effective April 28 to June 28

* Second short-selling ban during global crisis

(Adds background, details)

ATHENS, April 28 (Reuters) - Greece's securities regulator on Wednesday banned short-selling in shares on the Athens bourse until June 28 after investors responding to the country's deepening debt crisis ditched Greek assets a day earlier.

"The Capital Market Commission, having considered the extraordinary conditions in the Greek market, has decided to ban short selling on the Athens stock exchange. The rule will be in effect from April 28 until June 28," it said.

Greek shares .ATG fell 6 percent on Tuesday, with banks .FTATBNK plunging 9 percent. After the close Standard & Poor's cut Greece's credit rating to speculative grade.

It was the second time during the global economic crisis for the securities regulator to ban short sales on the Athens bourse. The first was in October, 2008 and was lifted in June last year.

Subsequent rules required that short-selling on the Athens bourse be flagged and meet an uptick rule.

Greek shares are down 22.7 percent year to date with banks nursing deeper losses of 36.9 percent.

S&P cut its rating of Greek government debt by a full three notches to BB-plus, the first level of speculative status. The outlook is negative, meaning the agency could downgrade Greece again. [ID:nLDE63P1EG]. (Reporting by George Georgiopoulos; Editing by Hans Peters)