US banking giant Citigroup said Monday it could plead guilty to an antitrust violation as part of a settlement of charges it helped rig the massive foreign exchange market.

Citigroup said it is in "active" discussions to settle the forex probe and that "a resolution with the Department of Justice could include a guilty plea on an antitrust charge," in a filing with the Securities and Exchange Commission.

A settlement in the forex rigging case between five banks, including Citigroup, and US and British regulators could be announced as soon as Wednesday, according to people familiar with the matter.

The size of the fine was expected to range by institution, with some banks paying hundreds of millions of dollars and the most egregious violators paying more than $1 billion.

Citigroup's penalties are expected to be among the biggest in the group, these people said.

The sprawling forex probe has ensnared most large banks and centered on accusations traders conspired through instant messages and online chats to manipulate the market in ways that cheated clients and bolstered their own profits.

Besides New York-based Citigroup, the other banks settling would be US bank JPMorgan Chase, British banks Barclays and Royal Bank of Scotland, and Swiss bank UBS.

Citigroup also said in the SEC filing it had been told by the US Justice Department that prosecutors do not intend to bring criminal charges against the bank in a parallel probe of a multi-bank conspiracy to rig London InterBank Offered Rate (LIBOR) interest rates, the reference for about $360 trillion in transactions around the world.