Its latest Annual Survey found almost a quarter (23%) of pension schemes are now shut to both new staff and to future contributions from people who were already in the pension.
This is up by a third from 17% in 2010, and was just 3% in 2008.
Among those pension schemes which are closed to new staff but still open to existing staff, 30% expect to close the pension altogether in the next five years. They plan to move staff into a defined contribution pension, where the employer is exposed to less risk.
One in 10 (11%) say they will keep the existing defined benefit pension scheme structure, but will make it less generous. This could include changing accrual rates or moving from a final salary to a career average structure.
The findings reflect an escalation in the decline of final salary (or defined benefit) pensions, as schemes that have already closed to new joiners shift their focus to existing members.
Final salary pensions have been increasingly strained by rising longevity, poor investment results, and red tape. Employers have been closing these pensions to try to manage risks and mounting costs. Only 19% of private sector schemes are now open to new joiners, compared with 88% ten years ago.
Joanne Segars, NAPF chief executive, said: "The private sector is seeing a seismic shift in its pensions, and more change is certain. Final salary deals are coming off the table and are either being watered-down or replaced altogether.
"Demographic and financial pressures mean businesses are struggling to afford these pensions. Many firms are trying to get a grip on the risks and rising costs by freezing the fund to both new and existing staff.
"While it is difficult to be exact, we estimate up to a quarter of a million have been moved out of their final salary pension over the past three years.
"People will often find that the replacement pension on offer is a good one. It's encouraging to see that, despite the harsh economic climate, payments into defined contribution pensions by staff and their employers have remained stable. Whatever the type of pension, the main thing is to get more people saving. The UK simply isn't salting enough away for its old age."
The NAPF Annual Survey, now in its 37th year, also found despite the economic downturn, total contributions from both employers and employees into 'defined contribution' pensions have remained stable at around 12% over the past five years.