The Thrift Savings Plan may allow investors to convert to Roth treatment money that was invested on a pre-tax basis, although there’s no certainty regarding when or even if that option will become available.

A provision in the “fiscal cliff” tax law allows, but does not require, tax-favored retirement savings plans such as the TSP to let investors make such a change. Investors would have to pay taxes on the amount converted.

That was one of the law’s provisions designed to produce tax revenue to help pay for delaying “sequestration” by two months. That automatic cutting in many agency programs is now set to begin in early March unless prevented.

In traditional investing, participants put in money on a pre-tax basis, and that money along with its earnings is taxable on withdrawal. In Roth investing, investments are made with after-tax money and are tax-free on withdrawal, as are the associated earnings if certain conditions are met.

The TSP said in a statement late Wednesday that it is “waiting for tax reporting guidance from the IRS and will be studying the actions required to offer a conversion option. After that review, we will make decisions on whether to proceed.”

Putting in place such an option “would not be a simple thing,” and there is no time frame for a decision, said TSP spokeswoman Kim Weaver.

One issue is that Roth balance earnings are tax-free only if five years have passed since the start of the first year in which that type of investment was made. That would require the TSP to keep track of money in a way that it does not monitor today, she said, and could trigger a need to revamp the agency’s computer systems.

After gaining the authority to offer Roth-style investing, the TSP needed more than two years, until May 2012, to make it available, largely because of record-keeping issues.

Another factor that could play into the TSP’s decision is the level of investor interest in making such conversions, given the potentially substantial tax bite.

As of late 2012, about 64,000 investors had Roth balances out of the 3.3 million federal employees and uniformed military personnel eligible to make such investments. About another 1.2 million account holders have separated for retirement or other reasons and are no longer eligible to make new investments.

On average, Roth balances made up only a small percentage of accounts, with traditional investments making up the large majority.