Billionaire investor Carl Icahn offered to underwrite a $6 billion loan to CIT Group Inc. (CIT) on Monday, complaining that a proposed solution hammered out by the cash-strapped century-old commercial lender and its largest creditors is too expensive and detrimental to CIT's smaller bondholders.

In a blistering letter to CIT's board dated Monday, Icahn--who refers to himself as CIT's largest creditor--said the company is "shamelessly" offering to sell certain large bondholders $6 billion of secured loans "well below their fair market value."

He said the offer to these large creditors is harmful to thousands of CIT's smaller bondholders and suggested it was made to encourage the large creditors to approve CIT's effort to reduce its debt load though a bond swap or prepackaged bankruptcy.

As an alternative to the current plan, Icahn offered to underwrite the $6 billion loan that he said would save the company as much as $150 million in fees to prospective lenders under the company's proposed financing.

More importantly, Icahn said in a statement that his offer wouldn't force bondholders to vote for CIT's current restructuring plan, which he claims would "entrench current board members and give them releases for a range of past acts."

CIT plans to ask Icahn for more information about his proposal, calling it the first time it knew of Icahn's interest.

In his statement, Icahn said CIT's board, which has "reigned over its ruin," is proposing a reorganization plan that "is designed to keep the existing regime and its handpicked successors in control" while also protecting the board from certain claims by stock and bondholders.

In exchange for committing to and funding the new term loan under the company's plan, CIT would pay lenders a total of 5% in commitment and funding fees, according to Icahn's letter. This would cost CIT $300 million in addition to an interest rate of at least 9.5%, he said. CIT originally received a $3 billion rescue loan from a steering committee of six of its largest bondholders in July.

Icahn built up his position CIT's debt over the last couple of months, according to one person familiar with the deal. Icahn declined to officially comment on the size of his investment in the firm.

Icahn said that if CIT rejects his loan offer, other banks would be eager to underwrite the financing, with large savings to the company as long as they weren't required to agree to CIT's restructuring.

Icahn has a reputation for opportunistically - and publicly - buying into companies in distress. He acquired the Tropicana Casino & Resort in Atlantic City earlier this year after the casino filed for Chapter 11 bankruptcy protection in 2008 and was among the syndicate of lenders in July that stumped up $500 million in debtor-in-possession financing to support Lear Corp.'s bankruptcy.

Stock investors seemed to welcome Monday's announcement from Icahn, but some CIT investors and bond traders were left scratching their heads by the move.

"This throws a significant monkey wrench into what the board is trying to do with the two-tier vote (on CIT's restructuring)," said Chris Munck, who trades CIT bonds at B. Riley & Co.

"The question is, what does Icahn's move mean from an equity standpoint? He's not going to offer this loan without getting some upside," Munck added.

CIT, which is teetering on the brink of bankruptcy, late Friday amended the terms of a sweeping debt exchange asking investors of around $31 billion in bonds to cut this debt by at least $5.7 billion and extend the debt maturities.

Bondholders are also voting on a prepackaged bankruptcy plan, which many see as the more likely outcome.

Under the revised terms, New York-based CIT will increase the amount of equity offered to holders of its subordinated debt and include bonds that mature after 2018 that previously weren't part of the exchange offer or reorganization plan that was announced Oct. 1. The company is also proposing that maturities on new notes issued as part of the exchange will be shortened by six months. It will also increase the interest on new debt offered to holders of bonds sold by a Canadian unit of CIT, the firm said on Oct. 16.

The company is also looking for new leadership after current CEO Jeffrey Peek said he will step down at the end of this year. While the changes outlined Friday make the debt exchange plan more palatable to some bondholders, they do little to address CIT's long-term outlook.

Shares of CIT were last up 9.8% at $1.23. The stock is down 73% this year.

-By Kate Haywood, Dow Jones Newswires; 212-416-2218; kate.haywood@dowjones.com

(Maxwell Murphy and Kevin Kingsbury contributed to this report.)