Updated from 9:30 a.m. EDT

Instinet Group

(INGP)

received a letter from Third Avenue Management, an investment firm based in New York, proposing the acquisition of its institutional brokerage business.

The Wall Street Journal

reported Wednesday that some Instinet shareholders, including Third Avenue, think the brokerage operation isn't fetching enough through a previous sale agreement. The offer is $307 million, according to the published report.

"We have received a letter from Third Avenue Management proposing to buy the assets and liabilities of one of our businesses, Instinet, The Institutional Broker," Instinet said in a press release. "The proposal described in the letter is subject to, among other things, the completion of due diligence to Third Avenue Management's satisfaction. Our Board of Directors is reviewing the letter in a previously scheduled meeting."

Instinet's statement continued, indicating that "under our merger agreement with The Nasdaq Stock Market, Inc., we are subject to restrictions with respect to third party proposals of the type contained in the letter from Third Avenue Management."

Earlier this year, the

Nasdaq Stock Market

agreed to buy Instinet. Under that pact, Nasdaq would buy Instinet's electronic trading platform, while the institutional brokerage unit would be sold to Silver Lake Partners.