** Possible Betfair-Paddy Power 5 bln stg merger a bad betfor rivals
** Merger could create one of the world's largest onlinebetting and gaming groups with estimated revenue of over 1.1 blnstg
** William Hill and Ladbrokes both down morethan 5 pct; worst performers on the FTSE 250 Index
** Playtech falls 2 pct while Bwin.party down 1.4 pct
** Betfair and Paddy Power only gambling cos in the green onthe STOXX Europe 600 Travel and Leisure index
** Possible merger marks the latest in a string of possibletie-ups across the sector
** Last month bookmakers Ladbrokes and Gala Coral sealed anall-share merger, while 888 and GVC are in abidding war for online gambling firm Bwin.party (Reuters Messaging:noor.hussain.thomsonreuters.com@reuters.net)
Playtech (PTEC)