A battle for the New York Stock Exchange has intensified with the co-bidders saying they will make a hostile takeover bid.
NYSE Euronext has twice rejected an unsolicited approach from Nasdaq and IntercontinentalExchange (ICE) worth about $US11.3 billion).
NYSE's board has already accepted an offer of $US10.2 billion from Deutsche Boerse.
But Nasdaq-ICE has said it will now make a formal offer direct to NYSE's shareholders later this month.
The BBC reports the move appears designed to press shareholders in the NYSE to force the board to the negotiating table.
NYSE has rejected Nasdaq-ICE's offer of talks, saying that the takeover approach is an ''empty vessel''.
Last month, Nasdaq-ICE said that they want to split NYSE's equities and futures businesses between them.
Late on Monday, Nasdaq, which runs equities markets in the United States and Europe, and ICE, an energy futures specialist, announced that formal details of their bid would be released in a prospectus this month.
The co-bidders said they were taking their bid directly to shareholders because NYSE's board had ''refused to engage us in discussion''.
The BBC reports the move could put pressure on Deutsche Boerse to improve its existing offer.
The Deutsche Boerse bid may also raise regulation issues, as a deal would create a massive presence in certain European financial products.
There is also some political opposition from those against the idea of a foreign company taking over a Wall Street brand.
NYSE shareholders are scheduled to vote on 7 July on the Deutsche Boerse plan.