NEW YORK (AP) — Struggling lender CIT Group Inc. warned Monday that it might still have to file for bankruptcy protection even if it completes a revised debt restructuring plan.

Billionaire investor Carl Icahn sent a letter to CIT Group’s board, offering to underwrite a $6 billion loan he says would save the lender $150 million, according a report in the Wall Street Journal. A spokesman for CIT Group wasn’t immediately available for comemnt.

Shares of CIT Group jumped 21 cents, or 18.8 percent, to $1.33 in early morning trading Monday amid reports about Icahn’s offer.

The New York-based lender to small and mid-sized businesses is trying to cut its near-term debt burden by $5.7 billion. On Friday, it sweetened an exchange offer to current bondholders as it tries to ensure the offer’s success.

The revised exchange offer from CIT Group gives bondholders a better interest rate and shorter maturities on the new debt. It also opens up the exchange to investors holding bonds that mature after 2018.

The revised exchange offer would give the government — which has already provided CIT Group $2.3 billion in aid — a bigger stake in the lender.

At the same time as it tries to swap current debt, CIT Group is asking bondholders to approve a prepackaged reorganization plan should it need to file for bankruptcy protection.

As one of the nation’s largest lenders to the retail industry, some economists say the company’s collapse could hurt a U.S. economy struggling to recover from recession.

Its customers range from Dunkin’ Donuts franchisees to department store operator Dillards Inc. It is also a short-term financier to about 2,000 vendors that supply merchandise to 300,000 stores, according to the National Retail Federation.

CIT Group’s losses have been mounting as its borrowing costs have outstripped its income amid the credit crunch. It received $2.3 billion in federal bailout money last fall and a $3 billion emergency loan in July from some of its largest bondholders.

The Treasury Department would receive a 5.4 percent stake in CIT Group under the amended debt exchange offer, compared with a 2.4 percent stake under the original plan launched at the beginning of October.

Current common shareholders will own about 2.5 percent of CIT Group if the exchange offer is completed. The remainder of the company would be owned by bond holders participating in the new debt exchange.

CIT had $54.09 billion in outstanding long-term borrowings as of June 30, including $13.85 billion due by June 30, 2010.

Amid uncertainty at the company, Jeffrey Peek, CIT Group’s chairman and CEO, announced last week he would retire at the end of the year. Peek, 62, has worked at CIT Group since 2003.