Managers blame the increased costs on people having longer retirements, falling interest rates and rising inflation, while employee contributions have stayed static.
BMW has put in £150m to cover a deficit in the pension scheme since 2003 will add £23.6m every year to remove the deficit by 2017.
It wants staff to consider options, including increasing contributions and cutting early retirement provisions.
But the move has drawn criticism from the Unite union. Dave Osborne, national secretary, said the proposals in effect amounted to a "draconian attack" on the pension scheme and employee's entitlements in retirement.