The US stock market operator was already the LSE's largest shareholder after building a 24.1% shareholding in the UK exchange.

The latest stock purchase is the fifth time Nasdaq has increased its holding in the LSE since buying a 14.99% stake in the UK market operator in April, less than two weeks after dropping its £2.43bn cash bid for the LSE.

In today's brief statement, Nasdaq says it bought the 2.2 million shares for 1235.5 pence each. LSE stock rose 1.50% to 1255.00 pence on the news.

Bob Greifeld, president and CEO, Nasdaq, says: "We are pleased to have increased our strategic investment in LSE in excess of the important threshold of 25%."

The news came as former LSE suitor Deutsche Börse outlined its proposal for a merger with pan-European exchange Euronext, which includes the establishiment of a new Netherlands-based parent company.

Deutsche Börse says the two exchanges would also have equal representation on the merged company's board. The group's headquarters and the majority of its executive management would be based in Frankfurt, although its cash equities business would be based in Paris, while derivatives trading would be located in Frankfurt and London.

A spokesperson for Euronext, which is rumoured to be in advanced negotiations with New York Stock Exchange (NYSE), told Financial Times reporters that the company was evaluating the different options for consolidation and would outline the most attractive to shareholders at its AGM next Tuesday.

But according to the FT report, Euronext shareholders have given the Nyse a deadline of next Tuesday to make a firm offer for European exchange or they will vote to support Deutsche Börse's rival bid.

Meanwhile, Italy's Borsa Italiana has also expressed interest in a tie-up with Euronext. Angelo Tantazzi, the chairman of Borsa Italiana, told reporters on that Euronext would be the exchange's first choice for an international alliance.