The system for 22-year-olds and older has been introduced gradually since October 2012.
Work and Pensions Secretary David Gauke told the BBC's Andrew Marr Show there had been "greater saving for pensions" since automatic enrolment came into effect.
Millions of pensioners have been left in "total confusion" thanks to a "contracting out" scheme - which means they no longer qualify for a full state pension.
These people paid a lower rate of National Insurance while working, in exchange for a lower rate of pension.
Every worker fitting this description begins saving into the pension unless they opt out.