China Southern Power Grid could also join the consortium, Expansion said.
Western Power is valued at around 14 billion euros ($17.07 billion) while Naturgy would invest less than 2 billion as a minority partner in the consortium, Expansion reported.
Two other consortiums of institutional investors are preparing to launch rival bids for Western Power, which supplies electricity to some 8 million people in Britain.
Naturgy declined to comment on the report. Allianz and PPL could not be reached for immediate comment.
($1 = 0.8200 euros)
(Reporting by Nathan Allen; editing by Jason Neely)