PARIS, May 3 — A long-anticipated round of mergers among European and New York stock exchanges took a tentative step forward on Wednesday as Nasdaq said it was increasing its stake in the London Stock Exchange.

The move by Nasdaq came as Euronext, the European exchange, said it was scrapping talks about a potential deal with the British bourse. The announcement put an end to 16 months of speculation about a tie-up between the London exchange and the Continental bourses operated by Euronext.

It also cast doubt on a possible deal between Euronext, which is based in Amsterdam but has its head offices in Paris, and Deutsche Börse, the largest German stock exchange.

Any merger of the London exchange and Nasdaq, or of Euronext and Deutsche Börse, would represent a significant step toward consolidation in a sector that remains fragmented along national lines, particularly in Europe.

For the London exchange, which has fended off several offers in recent years, a future with Nasdaq now appears inevitable despite its desire to remain independent, London analysts said. On Wednesday, Nasdaq announced the purchase of 9.79 million shares for £119.2 million, or $219.2 million, raising its stake in the London exchange to 18.7 percent, from 15 percent.

Euronext, which comprises the Dutch, Belgian, French and Portuguese stock exchanges as well as derivatives markets in London and other cities, faced strong pressure from some of its shareholders to abandon the idea of bidding for the London exchange, in part because it would have been an expensive purchase.

But analysts also said Euronext's decision to back away from bidding demonstrated the effectiveness of Nasdaq's strategy. Nasdaq's stake "was always going to be a deterrent," said Michael Long, an analyst with Keefe, Bruyette & Woods in London. Fending off other bidders "was part of Nasdaq's intention, and they've succeeded," he said.

Early last month, Euronext announced it would hold a vote on the principle of a tie-up with Deutsche Börse at its annual general meeting on May 23. But even that tentative step is far from straightforward.

Last year, merger talks between French-run Euronext and Deutsche Börse hit obstacles, including differences over where the combined group would be located and what computer systems it would use. This time around, tensions already are surfacing over what model the combined exchanges would employ to process and settle trades.

Euronext processes and settles trades through companies it does not control, LCH.Clearnet and Euroclear — a model strongly preferred by groups of British, French and Italian banks and by European Union single-market regulators. Deutsche Börse performs those tasks in-house, contributing to its overall profitability.

On Wednesday, Euronext called a merger with Deutsche Börse "a serious option" but it discouraged shareholders from signing up to a deal before they had all the facts about how the combined company would operate.

"Euronext is concerned not to restrict its flexibility during this period," the company said. Talks "are therefore continuing, with a view to identifying the most attractive and value-creating transaction available."

In another sign that consolidation may be under way, some shareholders in the Borsa Italiana, based in Milan, are encouraging a deal with Euronext rather than a sale of shares in the exchange that had been planned for this year.

Stefano Micossi, vice chairman of the Milan exchange's consultation committee, told Bloomberg News that he was "100 percent convinced that the best solution for Borsa Italiana is integration under the Euronext umbrella."

With Nasdaq now seemingly in a strong position to take over the London exchange, and with merger concessions from Deutsche Börse uncertain, Euronext could be shying away from a deal with either of the exchanges. A tie-up with the Milan exchange, however, may be welcome.

The role that the New York Stock Exchange could play in London or in Continental Europe is also unclear. In April, the New York exchange said it had recently held talks with rivals about an acquisition, but did not provide details.

Many London exchange users expect Nasdaq to raise its stake to at least 20 percent in the near future, because at that threshold it can count some of the London exchange's revenue on its own balance sheet.

Clara Furse, the chief executive of the London exchange, has not discussed Nasdaq's bid with her Nasdaq counterpart, Robert Greifeld. Ms. Furse wants the exchange to remain independent, according to several executives she has confided in. By putting off any face-to-face negotiations, she may also be driving the price higher, some analysts said.

"If I were Clara, I would go sit on the beach for a month, and have people wire in their latest bids," said Douglas Atkin, chief executive of Majestic Research, an independent research firm. "Not engaging is creating more of a frothy situation."

James Kanter reported from Paris for this article and Heather Timmons from London.