The London Stock Exchange’s (LSE) (LSE.L) share of United kingdom share trading is set to fall below 50 percent in April for your first time in its 210-year history, in part reflecting an exodus of high-frequency traders. The LSE completed the migration of its primary Uk order book Sets to faster technology in mid-February, three months later than planned, a move the exchange hoped would boost market share in part by luring high-frequency traders (HFTs).

But Sets’ Uk industry share has slipped to 49 percent in April after hovering just above 50 percent in recent months, Thomson Reuters data shows. And with barely two trading days left, this could be the initial month Sets has hosted less than half of United kingdom share trading.

The exchange is operating hard to diversify its revenue sources, such as through its deal to take above Canadian bourse operator TMX Group (X.TO), but the drop in the LSE’s major market share can be a setback for the British exchange, which lists domestic share trading as its top-earning enterprise line.

HFTs are hedge funds or proprietary trading firms that use state-of-the-art computers to generate millions of trades — an important source of buying and selling revenue for exchanges.

Traders cited the delayed technology upgrade, blamed in part on sabotage fears, as well as increasing trade in “dark pools”, where traders can move large orders without revealing themselves for the open or “lit” marketplace, for that slide in marketplace share.

But they also pointed for the gradual withdrawal from London by some HFTs, who are looking to other European markets such as Paris and Frankfurt because the United kingdom is saturated with speedy traders, making it harder to turn a profit.

“HFTs are struggling to make money on the LSE because from the already highly competitive nature of market-making on that exchange,” said the head of electronic investing at a single large European investment bank.