DETROIT -- The UAW's retiree health care trust plans to sell 40 million of its shares in General Motors, a move that could jeopardize the trust's seat on the automaker's board of directors.

The automaker late on Tuesday said the UAW Retiree Medical Benefits Trust, or VEBA, plans to sell the shares -- about 29 percent of its 140.15 million shares -- in one or more transactions through underwriters Citigroup and Barclays.

GM, according to a press release and filing with the Securities and Exchange Commission, intends to repurchase about $100 million of the shares at "the price per share that the underwriters pay." Based on GM's Tuesday closing price of $40.17, it would receive about 2.5 million shares.

The sale could put the VEBA's board seat at risk, as the trust needs to retain at least 50 percent of the shares it initially acquired as part of the automaker's emergence from bankruptcy, according to an October 2009 stockholder agreement between the UAW trust, GM and the U.S. Treasury.

It's unclear at this time if the initial shares outlined in the agreement include a three-way stock split prior to the automaker's November 2010 initial public offering that brought the trust's shares up from 87.5 million to 262.5 million.

If the 50 percent stake in its original shares included the split, the announced sales would drop the trust to 100.15 million, or 38 percent, no longer guaranteeing it a board seat. If it does not, the trust would have to drop below 43.75 million to risk loss of representation on the board.

GM, in its 2010 annual filing to the SEC, listed 263 million shares of common stock as the initial issued securities, while the 2009 agreement lists the 87.5 million.

A spokesman for the VEBA, which is the automaker's largest shareholder at about 10 percent, did not immediately respond for comment late Tuesday.

GM spokesman Tom Henderson said the automaker is currently focusing on assisting the trust in its sale, which also was part of the 2009 agreement.

"Right now, we're focused on assisting them with this offering," he said. "We'll focus on governance issues, including nomination of board seats, in the future."

The VEBA's seat on the board is currently empty following the abrupt resignation of former UAW Vice President Joe Ashton in December.

Ashton's resignation occurred a month after The Detroit News reported that federal investigators were "interested" in him and Cindy Estrada, his successor leading the union's GM department.

A source with knowledge of the investigation later confirmed that interest to Automotive News. Neither has been charged or formally named in the ongoing investigation.

The investigation initially centered on the misuse of funds from the automaker for a joint training center between Fiat Chrysler Automobiles and the union. It has since expanded to similar facilities between the UAW and GM and Ford Motor Co.

In November, both GM and Ford Motor Co. said they are cooperating with federal investigators who have subpoenaed information about jointly operated training centers with the UAW.

Thus far, four people -- two from FCA and two linked to the UAW -- have been charged and entered guilty pleas in the case, while several others have been charged but not yet identified in publicly available court documents.

Ashton, according to a VEBA spokesman, remained a member of the trust's board of directors as of Feb. 16. The board is made up of 11 members, including six independent seats and five appointed by the UAW.