NEW YORK (CNNMoney.com) -- Mortgage finance giants Fannie Mae and Freddie Mac were ordered by their federal regulator to no longer trade their shares on the New York Stock Exchange, the agency announced Wednesday. Both stocks plummeted on the news.

The Federal Housing Finance Agency (FHFA) and its predecessor agency have overseen the operation of Fannie Mae and Freddie Mac since September 2008, when they were both placed under conservatorship, a form of control similar to what is found in a bankruptcy process.

Since that time, the Treasury Department has poured $83.6 billion into Fannie Mae and $61.3 billion into Freddie Mac to cover losses on the trillions of dollars worth of mortgage-backed securities they own or guarantee.

Billions of additional losses are forecast in coming years. The government controls the majority of the shares of each firm.

FHFA said in a statement that the planned delisting is due to the weak stock price for both firms, and not due to any determination about a change in condition at the firms or decisions about their futures.

"A voluntary delisting at this time simply makes sense and fits with the goal of a conservatorship to preserve and conserve assets," said FHFA's acting director Edward DeMarco in the statement announcing the move.

Fannie Mae's shares have hovered near the $1 level that is the minimum required by the NYSE since they were placed into conservatorship, while Freddie Mac's shares have typically traded at less than $2.

Fannie Mae (FNM, Fortune 500) shares plunged nearly 30% Wednesday morning to about 66 cents while Freddie Mac (FRE, Fortune 500) fell about 20% to 98 cents.

Each firms' shares are eventually expected to trade publicly on the over-the-counter bulletin board, also known as pink sheet trading.