Deutsche Boerse AG (DBOEY) is not for sale to American buyers, a state lawmaker said Thursday, following speculation that Europe's biggest stock exchange could be a target after the collapse of its planned $30 billion merger with TheLondon Stock Exchange Group  (LNSTY) .

Thomas Schaefer, who serves as finance minister for the German state of Hesse, whose capital is Frankfurt and its the financial center of Europe's biggest economy, told reporters that U.S. exchanges "should not attempt" a play for Deutsche Boerse, arguing that while it is a "private company, it also has state responsibilities."

"The stock exchange authorities of Germany have to guarantee that if there is a change of owner, it has to guarantee that business has to continue uninterrupted as normal and it doesn't matter who makes an offer," Schaefer told reporters in London during a visit to lure business to Frankfurt after Britain's exit from the European Union. 

The European Commission formally blocked the proposed LSE/Deutsche Boerse merger Wedensday, saying the tie-up would have created a monopoly in fixed income clearing markets.

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"The European economy depends on well-functioning financial markets. That is not just important for banks and other financial institutions. The whole economy benefits when businesses can raise money on competitive financial markets." said Competition Commissioner Margrethe Vestager, adding the merger "would have significantly reduced competition by creating a de facto monopoly in the crucial area of clearing of fixed income instruments."

Deutsche Boerse chairman Joachim Faber said the ruling was a "setback for Europe, the Capital Markets Union and the bridge between continental Europe and Great Britain" and called the year-old proposal to combine the two exchange operators as "a rare opportunity to create a global market infrastructure provider based in Europe and to strengthen the global competitiveness of Europe's financial markets has been missed."

The EU decision marks the third time the Deutsche Boerse has failed to either buy or combine with the LSE, following unsuccessful attempts in 2000 and 2005. It is also be the sixth time a takeover or a merger with the LSE was undone, either by regulators or shareholders, in the past 16 years.

Deutsche Boerse shares at €84.23 each in Frankfurt Thursday after rising 0.78% on the session, giving the group a market value of €16.388 billion ($17.5 billion)

Editor's Pick: This story was originally published on Thursday March 30 at 12:47 British Summer Time.