New York ? Some of the hottest Internet companies of the 1990s boom tentatively agreed Thursday to pay investors at least $1 billion to settle allegations that the dot-coms’ initial public offerings of stock were rigged to benefit favored customers.

The more than 300 companies also agreed to cooperate in the continuing case against 55 brokerages accused of taking part in the secret deals, which allegedly were aimed at making big profits for the investors who were allowed to get in early.

The huge case involves 309 lawsuits filed by investors over IPOs between 1998 and 2000 at such former high-flying companies as Global Crossing, MP3.com, Ask Jeeves and Red Hat.

The dot-coms were accused of knowing about the deals and failing to disclose them.

Under the settlement, the companies guaranteed investors an overall payment of at least $1 billion, but will not have to pay anything if the brokerages settle for at least that much, said Melvyn I. Weiss, chairman of a committee of plaintiffs’ attorneys.

The dot-com companies’ insurers will cover the settlement, which is subject to court approval.

The plaintiffs say the investment banks that underwrote the IPOs including J.P. Morgan, Credit Suisse First Boston, Morgan Stanley and Smith Barney plotted to artificially inflate the value of IPO stocks.

Among the techniques was a practice called “laddering,” in which larger blocks of stock were allocated to investors who promised to take still more shares after the stock hit the open market.

Stanley Bernstein, left, of Bernstein Liebhard & Lifshitz, vice chairman of the plantiffs' executive committee, confers with the committee's chairman, Melvyn Weiss, of Milberg Weiss Bershad Hynes & Lerach, at a New York news conference. Lawyers for investors suing 55 brokerage firms for allegedly rigging initial public offerings of stock during the tech boom announced Thursday a tentative, partial settlement worth at least billion.

Also, analysts who worked for the investment banks were accused of boosting dot-com stocks by deliberately issuing favorable research reports.

In addition, some investors were allegedly charged inflated commissions on other trades.

The settlement was reached after more than a year and a half of negotiations, Weiss said.

“We have always been of the mind that the primary target in this case is the underwriting community,” Weiss said. “This gives us a huge booster shot in our case against them.”

The Securities and Exchange Commission and the National Association of Securities Dealers conducted an extensive investigation of Wall Street’s dealings in IPOs. Regulators have recommended strict limits on several questionable practices popular during the tech boom, including laddering.

Earlier this year, 10 of the biggest investment banks agreed to change their research and IPO practices in a $1.4 billion settlement reached with regulators.