NEW YORK -- Investor Carl Icahn offered to underwrite a $6 billion loan to CIT Group Inc., complaining that a proposed solution hammered out by the commercial lender and its largest creditors is too expensive and detrimental to CIT's smaller bondholders.

In a letter to CIT's board dated Monday, Mr. 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."

Carl Icahn, speaking at a dinner last year, said CIT's offer to its larger creditors is harmful to thousands of the lender's smaller bondholders.

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 filing.

As an alternative, Mr. 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.

CIT declined to comment but said in a statement it plans to ask Mr. Icahn for more information about his proposal, calling it the first time it knew of his interest.

Shares of CIT rose nine cents, or 8%, to $1.21, in 4 p.m. New York Stock Exchange composite trading. The stock is down 73% this year.

"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."

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

Mr. 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 fees, according to Mr. 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.

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

Mr. 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.

Mr. Icahn has a reputation for buying into companies in distress. He acquired the Tropicana Casino & Resort in Atlantic City, N.J., 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 filing.

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

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

The company is looking for new leadership after Chief Executive Jeffrey Peek said he will step down at the end of the year.

Write to Kate Haywood at kate.haywood@dowjones.com