Think of an exclusive poker game where the minimum bet is close to $1 million. The stock-trading version, “dark pools” that host blockbuster trades, are pulling in major money in Europe.

On June 24, 2016 – the day after the Brexit vote – just 3% of trades on the London Stock Exchange’s Turquoise shares platform were carried out in its dark pool via so-called block trades, mega-bets of around $800,000 or more.

Fast forward about four years to Nov. 9, 2020 – another momentous day for markets, when Pfizer  was the first COVID-19 vaccine developer to announce it had produced an effective shot – and 58% of trades were executed in these giant blocks.

The ballooning interest is evidence that U.S.-style block trading of shares is “becoming truly mainstream” among European institutional investors, according to Robert Barnes, global head of primary markets at LSE Group.

The LSEG is one of a handful of European exchanges trying to grab a slice of the action, having seen dark pools – traditionally run by investment banks or specialised platforms – siphon off growing trading volumes from their industry.

Pan-European exchange operator Euronext told Reuters it was preparing to launch its own business – Euronext Block – in coming months.

Block trades are so big – EU regulators stipulate at least 650,000 euros ($795,000) for large-cap stocks – they are, in practice, too costly for investors to make on mainstream exchanges where they can move the market and take a big hit on price.

Such “off-exchange” deals accounted for 4.2% of overall EU monthly trading volumes in February, up from 1.5% in January 2017, according to investment bank and research firm Rosenblatt Securities. Over that period, block trades in the more mature U.S. market eased from 9.6% to 6.7%.

Thus business is brisk for dark pools, which match buyers and sellers away from wider market and regulatory scrutiny.

Goldman Sachs told Reuters its European block-trading volumes in the first quarter of 2021 had already surpassed full-year levels for 2020, which itself had doubled from 2019. It declined to disclose volumes. Another top U.S. bank confirmed similar rates of European growth on condition of anonymity.

Among other leading venues, more than $7 billion worth of shares were traded on CBOE’s European block-trading platform in April, and more than $10 billion at Liquidnet, according to data from broker and analytics firm Instinet.

Jason Lenzo, head of trading at London-based Russell Investments, said if block-trading volumes continued to rise, investors had to adapt. Russell has increased its share of block trades from 30% of all trade volumes a couple of years ago to the “mid-30s” now, he added.

The FTSE MIB climbed up by 0.29% to 25,452.33. In the cash markets, the DAX futures Germany was trading up 0.19%  to 15,632.17. CAC 40 futures in France fell by 0.21% to 6,507.60 while the FTSE 100 futures in the U.K. was up by 0.61% to 7,064.93 ,at the time of writing.