Pension provider Friends Life will not allow its customers to take partial withdrawals from their pensions.
The firm previously said savers would be able to make withdrawals when they liked, but it has now written to customers saying this would not be possible.
Friends Life started sending letters out to customers two weeks ago and it said around 1,200 to 1,300 customers would be affected.
The firm said the "vast majority" of queries came from customers wanting full cash withdrawals and it had decided to focus on this option. It could not provide a figure for the number of such queries.
Friends Life said it would offer partial withdrawal to customers "in due course" but could not provide a timescale of when this would happen.
The firm's action means customers who want to withdraw cash would need to make a full withdrawal of their pension fund as a lump sum.
This means those customers will get the first 25% tax-free, but the rest will be taxed at their marginal rate.
Friends Life is now part of Aviva, but the firm said only customers in the pre-merger business would be affected.
A spokesman for Friends Life said: "We apologise to those Friends Life customers who wish to partially withdraw their savings through the new pension freedoms as we are not offering this service at the moment.
"We have seen a high number of pension freedom enquiries from Friends Life customers, the vast majority requesting full encashment and this continues to be our focus. We are planning to offer partial withdrawals in due course. This matter affects customers in the pre-merger Friends Life business, and not other Aviva customers."