Nasdaq launched a £2.7 billion takeover bid for the London Stock Exchange (LSE) today.
The New York exchange offered 1243p a share for LSE as stock markets around the world look to consolidate.
Nasdaq built up a 25% stake in LSE in April and acquired a further 7,065,984 shares today for 1243p, or £87.8 million, to take its holding to 28.75%.
Nasdaq president and chief executive Robert Greifeld said he hoped to meet LSE chairman Chris Gibson-Smith today to discuss the offer.
Mr Greifeld said the offer was "full, final and fair" and added that he wanted the approval of the LSE board.
Shares in LSE opened nearly 4% higher at 1262p today.
The stock lifted as much as 124% this year to more than £13 a share amid takeover interest from Nasdaq as well as European rivals Deutsche Boerse and Euronext and Sydney-based investment bank Macquarie.
But LSE fell 7% to 1218p last week after Citigroup and Goldman Sachs said they were among seven investment banks planning a rival platform to challenge the likes of LSE.
It was feared that the move - codenamed Project Turquoise - could hit revenues at LSE and the subsequent fall in share price pushed it back within reach of Nasdaq.
Mr Greifeld said: "We believe that the price reflected in the market was in line with our view of the fully priced value of the exchange.
"We also believed that with the developments in the market and Project Turquoise it became clear that while the LSE has done well as a stand-alone entity for some time, in future years it is important for consolidation to take place."
He said Nasdaq had worked in a competitive environment since its formation 35 years ago and the prospect of competition in London was positive.
Nasdaq failed in an earlier attempt to take control of the LSE when LSE chief executive Clara Furse rebuffed a £2.4 billion 950p-a-share bid in March.
Today it said it planned a dual listing in London and New York if the offer is successful. There had been fears that a takeover could diminish the role of the City.
In a move to allay fears that the London market would be stifled by US regulation, Nasdaq said the LSE would still be regulated by the Financial Services Authority and would retain its own brand.
It said: "Nasdaq respects the strengths of the London market and intends that LSE's operations should continue in a manner consistent with current practice."
The tech-laden heavy exchange said its final offer would not be revised, unless another other party makes an offer.
The deal would create the world's largest equity market by listings made up of more than 6,400 quoted companies with a total market capitalisation of £6.3 trillion.
Mr Greifeld said: "We are excited about the prospect of combining two strong businesses to form the leading global, cross-border equity market platform giving issuers the ability to dual-list simultaneously in London and New York.
"The combined entity will be well positioned to lead further consolidation and compete effectively for the benefit of all market users."Reuse content