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The London Stock Exchange (LSE) has agreed a merger with TMX Group, which operates the Toronto Stock Exchange.

The merged group will keep headquarters in both London and Toronto and become the world's largest exchange for mining companies.

More than 6,700 companies will be listed on the combined exchanges, with a market capitalisation of about £3.7 trillion ($5.9tn), the firms said.

NYSE Euronext and Deutsche Boerse later said they were in talks about a merger.

That deal, if it went ahead, would create one of the world's biggest exchanges. It would also be a major competitor to the LSE and TMX.

Before those discussions were announced, shares in the LSE climbed sharply to close up 3.1%.

A number of leading exchanges have merged in recent years.

The New York Stock Exchange has bought Euronext, while Deutsche Boerse has taken over the International Securities Exchange in the US. The LSE, which has successfully fought off a number of takeover approaches, has itself bought Borsa Italiana.

And on Wednesday, trading in shares of both NYSE Euronext and Deutsche Boerse were halted amid speculation of a tie up - though both firms declined to comment.

The LSE's chief executive Xavier Rolet, who will head the new enlarged business said it was "an incredibly exciting merger with considerable growth opportunities".

"We are creating the world's largest listings venue for the commodities, energy and natural resources sectors, as well as the premium market for small, mid-size and growth companies."

TMX's chief executive Thomas Kloet will be the firm's president.

"We are creating an international group with deep expertise, undeniable leadership in key sectors and the ability to compete and win on the global stage," Mr Kloet said.

"Canadian customers will benefit from access to one of the world's deepest capital pools while European issuers will have an effective gateway to North American financial markets."

"These are arguably two of the most important mining exchanges in the world," Douglas Porter at BMO Capital Markets told the Â鶹ԼÅÄ World Service.

"Of course some of the world's biggest mining companies are on the LSE, and some of the world's biggest gold companies, and hundreds if not thousands of smaller mining companies are in the TMX Group.

"So certainly it brings together some of the most important mining groups in the world under one broad umbrella."

As well as the Borsa Italiana, the LSE also operates MTS, a European bond market, and the Turquoise trading platform. TMX also operates the Montreal Exchange and the Boston Options Exchange, among others.

The merger was "interesting" said Justin Urquhart Stewart of Seven Investment Management, but he added there may be opportunities for the LSE to be involved in more lucrative deals.

"What this would do is take them from 10th or 11th in the international league to fifth or sixth," he told the Â鶹ԼÅÄ.

"It'll allow them to have greater pools of liquidity and develop derivatives, but it's not the big story. The big story would be a potential linkage with one of the Indian markets, or potentially Singapore or maybe China.

"That's where they will get into a much larger scale operation. As yet we don't know anything, but watch India over the next few months."