This is the first time the proportion has fallen below 50% since the Office of National Statistics research started in 1997, when the figure was 55%.
The figures show 33% of private sector employees were saving through workplace pensions in 2011.
Towers Watson says there has been a significant decline since 2002, when private sector pension scheme membership was estimated to be 47%.
John Ball, head of UK Pensions at Towers Watson, said: "Pension participation has been getting worse for some time, but we should now be approaching the nadir. Beginning later this year, employees will be put into pension schemes automatically, so they are saving for retirement unless they actively choose to opt out. This will be a game-changer but it is being rolled out very slowly - for almost half of the people due to be auto-enrolled, nothing will happen for at least three years - and the minimum level of contributions has been set very low.
"Automatic enrolment sounds like a simple solution but the rules, which have only just been finalised, are far from straightforward. The largest employers have just a few months to finish overhauling their payroll and pension processes so they can comply.
"These employers typically offer valuable pension contributions already. An important question for them will be whether the employees they sign up automatically will value the pension as much as employees who actually opted in. Companies will want to avoid spending money on benefits that employees do not value. Some will want to communicate how much better than the minimum level their pension is. Others may automatically enrol staff at lower contribution rates and give them the option to trade up."
Just 9% of private sector employers were building up new DB pension entitlements in 2011, down from 11% in 2010 and 34% in 1997. By contrast, 79% of public sector employees were active members of DB schemes in 2011 according to the Annual Survey of Hours and Earnings data.
Darren Philp, NAPF policy director, added: "We've passed an important and worrying landmark. Less than half the workforce is now saving into a pension, and with people living longer the UK is facing a growing headache in paying for its old age.
"It is encouraging that saving has held up in the public sector. But it is a worrying picture in the private sector, which has seen a significant fall in pension uptake.
"While reform is coming to the public sector, the private sector has already seen a seismic shift in its pensions. Over the past decade, many final salary deals have come off the table to be replaced with newer 'money purchase' pensions.
"Sadly, the fall in people saving into a final salary scheme has not been fully matched by interest in other types of pension. The weak economy and falling confidence in financial products have also spurred many private sector workers to quit pensions altogether.
"Upcoming reforms to automatically put all workers into a pension will be a huge help in tackling the UK's savings crisis, especially in the private sector. The reforms could bring between five and nine million people into a pension, including younger people and many part-time workers.
"But more needs to be done. We must all work to provide pensions that people have the confidence to save in. This means rebuilding trust and confidence in the pension brand, and demonstrating what value people can achieve by saving in a pension."
Ball said: "The number of private sector employees in defined benefit schemes has been in freefall and will drop further before it levels off. Most companies' defined benefit schemes are closed to new entrants, so the number of employees with this sort of pension will gradually decline as existing staff move on. Many schemes are also about to embark on new valuations which may show bigger shortfalls. This could be a trigger for companies to stop existing members from accruing further benefits so that more of their limited pensions budget can go towards plugging the deficit.
"The Government has insisted that, following the Hutton reforms, defined benefit pensions in the public sector are here to stay for at least another 25 years. For the overwhelming majority of private sector employees it will be a very different story. As the spread of defined contribution provision puts pension risks firmly on individuals' shoulders, employees must be encouraged to take responsibility for their retirement planning and consider upping their contributions if blown off course. If everything is left on autopilot until retirement, the only things that can adjust are when you can retire and how much you have to live on when you stop working."
