Spanish power co's slip after Congress blocks law to strengthen electricity system
** Shares in Spanish power companies slip after the lower house of Spanish parliament rejected a law aimed at strengthening electricity system after April's power grid blackout ** Utilities Endesa ELE.MC, Naturgy NTGY.MC, Redeia REDE.MC and Enagas ENAG.MC fall between around 1% and 2% ** The stocks underperform Spanish blue-chip IBEX35 .IBEX index, which is up 0.8% (Reporting by Joao Manuel Mauricio) ((JoaoManuel.VicenteMauricio@thomsonreuters.com))