Inside Barwon Prison. Credit:Angela Wylie

A huge prison expansion is being planned west of Melbourne to cope with expected increases in the number of inmates.

The expansion will roughly double Barwon Prison, but the Andrews government will not reveal its cost until the budget is released later this year.

The announcement on Thursday morning is the latest volley in the battle over law and order, which the Coalition believe will be vital to them winning government at the November election.

The plan has been dubbed the Lara prison precinct expansion, but is unclear how many new beds will be added.