The inexplicable volatility that roiled the British pound last week came as no surprise to Bank of America Corp., which just days earlier warned that liquidity in the $5.1 trillion-per-day global currency market was far worse than anyone imagined.

Sterling sank 6.1 percent in a span of minutes in early Asian trading Oct. 7, following at least three other bouts of puzzling foreign-exchange turbulence in the past two years. The latest episode involved the fourth-most-traded currency, serving up a stark reminder of the pitfalls that investors face in the world’s biggest financial market as banks — the traditional middlemen — step back amid post-crisis regulations.

The currency market is growing more fragile because “phantom liquidity” is undermining investors’ ability to buy and sell when they need to, Bank of America analysts wrote in an Oct. 5 note. Their research found that with trading volumes in decline, transactions have a 60 percent greater impact on prices than just two years ago.

The size of last week’s move “in the deepest and biggest market in the world scares the hell out of everyone,” said Bob Savage, chief executive officer of hedge fund CCTrack Solutions LLC in New York. “The larger story is whether there’s anything to do about it.”

Stepping Back

The lifeblood of the global economy, the currency market has been blighted by staff reductions and a rate-fixing scandal that’s ensnared some of the world’s biggest banks. Stricter regulation enacted since the financial crisis has prompted dealers to step back, opening the door for automated market makers to expand, though the new entrants haven’t stemmed a drop in volumes. Daily trading in global currencies has shrunk almost 6 percent since 2013, Bank for International Settlements data show.

The pound’s so-called flash crash follows a similar event for South Africa’s rand, which in January tumbled 9 percent in 15 minutes, before rebounding. New Zealand’s dollar had its own flash crash in August last year.

“If investors have an urgency to transact, the liquidity that they see might disappear and they might not be able to do so,” said Vadim Iaralov, a foreign-exchange strategist in New York at Bank of America, the world’s fifth-largest currency trader according to Euromoney magazine. “There may be nobody on the other side of the trade.”

Shock events may trigger unexpected volatility and cause bid-ask spreads to widen significantly as market makers withdraw during turbulent periods to avoid losses, Iaralov and co-author Christopher Xiao wrote in last week’s note.

Liquidity’s Cost

For strategists at JPMorgan Chase & Co., the second-biggest currency trader, the pound’s plunge breathes new life into the debate about the market’s changing structure and its impact on liquidity.

In a sign of the growing competition from non-banks, computerized trading firm XTX Markets Ltd. placed ninth in Euromoney’s ranking of foreign-exchange traders, released in May. High-speed market makers have more than tripled currency volumes in the last three years, according to Aite Group, a consultant in Boston.

“The advent of non-bank liquidity providers such as HFTs has reduced bid ask spread and increased market efficiency in FX markets, but at the cost of lower market depth and withdrawal of liquidity provision in periods of stress,” JPMorgan strategists including Nikolaos Panigirtzoglou wrote in an Oct. 7 note.

2016 Laggard

The pound has already fueled plenty of handwringing.