People who choose to opt out of workplace pension saving could be "throwing away" a potential £450,000 pot by the time they retire, an insurer has calculated.
Aegon made the calculation ahead of an increase in minimum contribution rates into workplace pensions set to come into force in April, with concerns some savers may choose to drop out.
The landmark automatic enrolment scheme has so far been seen as a success with nine in 10 people staying in their workplace pension rather than opting out.
From April 6, minimum contribution rates will increase from a 2% total including employee and staff contributions to a combined 5%, with a minimum of 2% from the employer and the remaining 3% from staff.
In April 2019, the rate will increase again, to 8%, with a minimum of 3% from the employer, leaving a 5% minimum staff contribution.
Minimum contributions are gradually being increased to help encourage people save enough for a comfortable retirement - but there have been concerns that some people may decide to drop out when rates increase.
Aegon said a survey of more than 700 people had found more than half (53%) were unaware of the imminent increase.
But it said workers should bear in mind that opting out means they will miss out on "free money" in the form of employers' contributions and tax relief.
Over time, their contributions plus this free money can grow to very substantial sums - and someone on average earnings who chooses not to join their workplace scheme at age 22 could potentially lose out on a fund of £450,000 by the time they reach 68, it said.
Aegon's calculation was based on minimum contributions and also made some assumptions about investment growth.
Kate Smith, head of pensions at Aegon, said: "Raising awareness of the benefits of pension saving is key to the success of an employee's financial future and we want to encourage employers to get across the value of workplace saving."
She continued: "The decision to opt out could end up costing workers up to £450,000 over their working lifetime, an enormous sum of money that would effectively be thrown away - why would anyone give up on money they're entitled to?"
Tom McPhail, head of policy at Hargreaves Lansdown, said typically someone needs to be saving around 12% to 15% of their monthly income throughout their working life to avoid a significant drop in their standard of living in retirement.
He said: "The simple message for employees is keep saving for your retirement if you possibly can and save as much as you can."