The option will come with eligibility restrictions and fees, and several financial advisers who have counseled federal employees about their TSP accounts cautioned that the opportunity should be handled with care.

The June 1 date was announced at the monthly meeting of the program’s governing board, which previously had said only that the option would become available sometime in June.

The expansion has been in development for years, since a law passed in 2009 to authorize the more broad options for TSP, which has more than 6.5 million account holders and some $740 billion on investment as of the end of April. It is part of an upgrade to the TSP’s operating platform that also adds new online services and security protections and a mobile app.

“For those of us who have been along this entire journey, it’s a really special moment,” acting board chairman David A. Jones, a financial industry veteran, said at the meeting. “We had a vision of what the TSP should be … it’s very much in line with what our hopes were.”

The mutual fund window will be available only to those with at least $40,000 in investments because of the combination of two restrictions: The initial investment through the window will have to be for at least $10,000, and no new investments can bring the outside share above a quarter of an account’s total.

About 2.3 million of the 4.1 million current and former federal workers in the TSP have balances above that threshold, while that is the case for only about 350,000 of the nearly 2.5 million current and former military personnel with accounts.

Also, because the 2009 law required that users of the mutual fund window bear its costs, they will have to pay annual fees totaling $150 plus a $28.75 per-trade fee.

“We’re not trying to put up a hurdle, but we’re trying to make sure people have thought about it and researched it a little,” TSP spokeswoman Kim Weaver said. “We wanted to make sure the amount that people were moving through was substantial enough so that it wouldn’t be substantially affected by the fees. We also wanted to make sure that people who were using it have some investing experience.”

The TSP traditionally has offered only five funds tracking broad stock and bond indexes and government securities, plus funds that mix shares of those funds in differing ratios depending on the expected date to begin withdrawals. Ahead of the upcoming change, the most significant expansion of options had been increasing the number of target-date funds from five to 10 in 2020.

Numerous bills have been introduced in Congress over the years seeking add investment funds to the TSP focusing on certain market sectors or excluding investments in certain types of companies. On its own initiative, the program had planned to widen its fund tracking international stocks to include markets of some two dozen more countries in 2020, but it retreated after political opposition arose because that expansion would have included stocks of Chinese companies.

Weaver said the TSP expects that after several years, it will reach a steady state of 2 to 3 percent of investors investing in outside funds. “There will be people who have been eagerly awaiting this, and there will be others who will look into it and decide later,” she said.

“This is going to open it up to a lot more choices. That’s great,” said Ian Arrowsmith, managing partner of Scarborough Capital Management in Annapolis, Md. “However, people have to be careful making the right investment choices for their age, their risk tolerance and their time horizon for when they’re going to retire.”

Jim Musgrave, a financial adviser with Research Financial Strategies in Rockville, Md., said he has seen many federal employees transfer their TSP account money into IRAs after retirement to get a wider range of investment choices.

“I think it’s very advantageous for participants to have the option to go outside of the funds in the TSP,” he said. “There’s a lot of people who are more sophisticated who would love the opportunity to go beyond those funds.”

“The downside is that a lot of investors chase returns,” he said. ‘They’ll say, ‘This sector did really well the last six months. I’m going to put my money there.’ That hot sector is eventually going to die down and something else is going to replace it. They’ll be heavily weighted in a sector when they should have started to trim it.”

Eligible investors will have access to the window through their online accounts, which will include a screening tool allowing them to select mutual funds meeting criteria they choose, including what the funds invest in and what fees they charge.