It said 1,153 companies have now met their legal obligations in auto-enrolling more than one million workers.
However, Hargreaves Lansdown said these companies employ 1.7 million workers who aren't enrolled because they earn under £9,440, are under the minimum age of 22 and are over the state pension age.
Part-time workers are the biggest concern and may get left behind in the retirement race. It said women are particularly at risk as three-quarters of part-time workers are female.
Laith Khalaf, head of corporate research at Hargreaves Lansdown said: "Auto-enrolment is a vitally important project, but it is not a silver bullet.
"Millions of part-time workers are going to be overlooked because they don't earn enough, likewise the self-employed are excluded."
He added: "There is still therefore a pressing need to foster a savings culture alongside auto-enrolment, and to provide a decent level of state pension."
