JOE SPAGNOLO POLITICAL EDITOR PERTHNOW JANUARY 24, 2015 6:00PM

PREMIER Colin Barnett’s plan to legislate a supersized City of Perth is set for the scrap heap, with members of his own party vowing to vote against it.

Mr Barnett’s former police minister, Rob Johnson, declared he would oppose the proposal when it was debated in State Parliament.

Mr Johnson said other Liberals were lining up to cross the floor, with Upper House MP Simon O’Brien speaking out against it.

In the Lower House, 21 Labor MPs, seven Nationals MPs and now Mr Johnson are set to join forces to try to defeat the Bill in the 59-member Legislative Assembly.

If just one more Liberal MP crosses the floor on this issue in the Assembly, the Bill will be defeated.

Mr Johnson said the City of Perth Act, which involved ­expanding boundaries to incl­ude major institutions such as the University of…