There were 2.9 million private sector workers placing money into schemes last year, marking the first time active membership has dipped below three million, an Office for National Statistics report said.
The decline pushed the overall number of active members in both private and public sector schemes down for the third year in a row in 2011, to 8.2 million.
The report comes just days ahead of the Government's landmark automatic enrolment scheme to tackle the pension savings crisis – which begins next month with the largest companies.
Active pension scheme membership has been in general decline since 1991 and there are four million fewer members of schemes in total compared with a peak of 12.2 million workers in 1967.
The Occupational Pension Schemes Survey said the recent drops reflect a fall in active membership of private sector defined benefit (DB) schemes, which has more than halved from 4.6 million in 2000 to just 1.9 million last year.
Many "gold-plated" private sector DB schemes – which offer a guaranteed pension based on earnings and length of service at the end of an employee's career – have been closed to new members as employers have found them increasingly expensive to run because people are living longer.
However, DB schemes are the norm in the public sector, which has seen active membership increase.
There were around 5.3 million active members of public sector schemes in 2011, an increase on 4.2 million in 1991.
When the records began in 1953, active membership of private and public sector pension schemes was roughly equal, at around 3.1 million people in each sector.
Ros Altmann, director-general of Saga, said the figures reflect a fall in trust and confidence in pensions, fuelled by scandals, disappointments for people whose pensions have not turned out as they expected and low annuity rates which have "left many pensioners receiving much poorer value for their pension savings than ever before".
She said: "The figures suggest many workers who have already been offered company pension schemes are voting with their pockets and choosing not to bother with pensions.
"The UK pension system is the most complex in the world, comprising many different bits of pension which almost nobody understands.
"Even leaving aside the difficulties of choosing good investment options, the pensions industry is full of jargon that most mere mortals have little hope of understanding."
From October 1, the largest employers, with 120,000 or more workers, must place eligible employees into workplace pension schemes, with firms gradually being enrolled in a staging process which will end in February 2018.
The reforms aim to address concerns that workers see pensions as a turn-off, at a time when people are struggling to put any money aside as wage increases fail to keep up with living costs.
Joanne Segars, chief executive of the National Association of Pension Funds, said: "The ongoing slide in pensions uptake in the private sector is deeply worrying.
"Our society isn't saving enough for its old age and millions of workers will end up scraping by on just the state pension.
"The decline of final salary pensions in the private sector is one factor behind this fall, but squeezed household budgets and the weak economy mean many people see a pension as a luxury rather than a necessity."
The Tory-LibDem Coalition said the new auto-enrolment measures would see more than half-a-million extra people saving in a workplace pension by Christmas.
By May 2015, an estimated 4.3 million more people will be saving for their retirement.
Iain Duncan Smith, the Secretary of State for Work and Pensions, described the changes as vital.