Intercontinental Exchange (Ice) confirmed it was mulling an offer for the London Stock Exchange (LSE), although it stressed it has not approached the group's board and has yet to decide whether to make a bid.
The LSE, which owns index compiler FTSE, last week revealed a proposed "merger of equals" with Germany's Deutsche Borse.
But the pair revealed their tie-up would see Deutsche Borse shareholders own more than 54% of the combined group and lead to the departure of LSE chief executive Xavier Rolet.
The LSE saw its shares jump to a record high of £29.14 at one stage after Ice revealed its bid interest, surging by around 8%.
Ice now has until 5pm on 29 March to make a firm bid or walk away under city takeover "put up or shut up" rules.
Its plans could scupper the LSE and Deutsche Borse merger, which already marks a third move by the pair to join forces after two previous failed attempts in 2000 and 2004-5.
But the LSE said talks with Deutsche Borse "continue to progress", adding it had not yet received a proposal from Ice.
Their possible merger is also under a cloud of uncertainty cast by Britain's EU referendum, with the LSE and Deutsche Borse warning last week it could be derailed in the event of a vote for Brexit.
The bourses said a Brexit "would put that project at risk" and have no w formed a referendum committee to assess the outcome of the EU referendum on 23 June.
Their proposed all-share deal would see LSE shareholders hold 45.6% of the group and Deutsche Borse the remaining 54.4%.
It would see Deutsche Borse boss Carsten Kengeter become chief executive of the combined company and LSE's Donald Brydon taking up the role of chairman.
The current chairman of Deutsche Borse Joachim Faber would become deputy chairman and senior independent director, with LSE's David Warren retaining his position as chief financial officer.
The combined business will run headquarters in London and Frankfurt.
The LSE is one of the world's oldest stock exchanges and can trace its history back more than 300 years.
The wider LSE Group was formed in October 2007 when the London Stock Exchange merged with Milan stock exchange Borsa Italiana.
It has completed a series of deals since then, the biggest of which was its £1.6 billion takeover of American stock index and asset management business Frank Russell in 2014.
Ice - which is headed by founder, chairman and chief executive Jeff Sprecher - has embarked on an acquisition spree in recent months, including a £3.7bn deal to buy financial market data provider Interactive Data Corporation.