European regulators have killed off the London Stock Exchange Group's £21 billion merger with Deutsche Borse, saying the deal would have forged a "de facto monopoly".
The European Commission moved to block the deal after it said the two exchanges had failed to address its competition concerns.
The move comes after the LSE rejected the commission's request last month to offload its 60% stake in the Italian trading platform MTS.
Margrethe Vestager, the EU's competition commissioner, said: "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.
"The merger between Deutsche Borse and the London Stock Exchange would have significantly reduced competition by creating a de facto monopoly in the crucial area of clearing of fixed income instruments.
"As the parties failed to offer the remedies required to address our competition concerns, the Commission has decided to prohibit the merger."