Occupational pension scheme membership reached a record high of around 45.6 million in 2018, according to official figures.
It compares with 41.1 million in 2017 and is the highest level recorded by the occupational pension schemes survey, the Office for National Statistics (ONS) said.
Automatic enrolment into workplace pensions, which started in 2012, has turbo-boosted the numbers of people who are now in the habit of saving for their retirement.
Around nine in 10 people automatically enrolled have chosen to stay in their scheme, rather than opting out – a much higher proportion than was initially expected.
But pension experts commenting on the ONS report raised concerns that many people are still contributing low amounts into their pots.
The average contribution rate for private sector defined contribution (DC) pension schemes last year was 5% of pensionable earnings, split between scheme members and their employers.
The minimum amounts that can be put into a workplace pension have gradually been phased upwards in recent years.
Helen Morrissey, a pension specialist at Royal London, said: “Thanks to auto-enrolment, membership of UK occupational pension schemes has hit an all-time high.
“But almost all of the growth is in people with an average of 5% of pay going into their pension pot.
“This compares with those in old-style final salary pensions who are getting 25% in total contributions.”
The figures also show that active membership of occupational pension schemes, which is generally made up of current employees still paying into their pots, reached 17.3 million last year.
The increase in active membership has been driven particularly strongly by private sector, the ONS said – and likely to be due to the impact of automatic enrolment.
Active scheme membership in the private sector has surged from 2.7 million to 11 million between 2012 and 2018.
Active membership in the public sector has increased from 5.1 million to 6.3 million over the same period.
Tom Selby, senior analyst at AJ Bell, said: “The first stages of the auto-enrolment revolution have been a success, with pension scheme membership now at record levels and inertia proving a powerful force as opt-outs remain low.
“The job is not done, however. While getting more people to pay into a pension is clearly a big step forward, the amount people contribute needs to increase substantially if we are to avoid a retirement crisis in the coming decades.”
He said: “A 25-year-old earning £30,000 and saving at the minimum would end up with a pot of around £200,000 in today’s prices by age 65 – that would buy a guaranteed, inflation-protected income worth about £6,500 at the moment.
“If you combine that with the state pension you’ll be living on around £15,000 a year.”
Tom McPhail, head of policy at Hargreaves Lansdown said: “For millions of people, payments into defined contribution pensions still need to be around double their current rates.”
Minister for Pensions and Financial Inclusion, Guy Opperman, said: “These figures show, once again, just how successful the Government’s pension reforms have been.
“They tell a story of millions of people now building a pension pot for their future and looking forward to a better retirement.”
Further reforms in the pipeline include pensions dashboards, which will enable people to see all their pots in one place online, and new collective defined contribution pension schemes aiming to offer potentially millions of workers better returns and healthier pension pots.