A “fat-finger error” by a London-based trader at Citigroup that triggered a flash crash across European stock markets could cost the bank at least $50m (£39.8m).

The trader was working from home last month when they incorrectly added an extra zero to a trade, Bloomberg reported, quoting “people familiar with the matter”.

That split-second mistake, on the 2 May bank holiday, wreaked havoc on markets across Europe, triggering a sell-off that reportedly wiped out as much as €300bn (£256bn) at one point.

Trading was briefly suspended in several markets that morning after leading share indices recorded sudden steep declines.

One of the hardest hit was Sweden’s benchmark OMX Stockholm 30 index, which fell by nearly 8% but later recouped most of the losses and ended the day 1.9% lower.

The Stoxx Europe 600 index of Europe’s leading shares lost as much as 3% before closing down 1.5%.

Even though the error was made by a London-based trader, the UK stock market escaped that day’s carnage as it was closed for the bank holiday.

Flash crashes, or brief price collapses, can be caused by human error. Although many aspects of trading have been digitised and automated, parts of the process are still manual, leaving room for such error. So called fat-finger blunders – where the details of a trade are mistyped – have become more common with the rise of high-frequency trading firms.

Until now, few details had emerged about last month’s incident. New York-based Citigroup owned up to the mistake in a statement a few hours later, saying: “This morning one of our traders made an error when inputting a transaction. Within minutes we identified the error and corrected it.”

Bloomberg has uncovered more since. It reported that the bank may record a loss of at least $50m after the incident, though work was still being done on this and the final figure may be higher.

It said the trader was part of the firm’s Delta One trading unit in London and had since been placed on leave while the bank reviewed the incident.

UK regulators are likely to have taken an interest, and the fact that the trader was working from home may be a focal point of any investigation.

However, the report quoted insiders as saying Citigroup took the view that it was human error unrelated the fact the staffer was working from home.

The Delta One unit sells financial products to sophisticated investors such as pension funds, hedge funds and blue-chip corporate clients.

Citigroup has past form. In 2020 it accidentally wired $900m of its own money to creditors of the cosmetics group Revlon.

One of the biggest flash crashes happened on 6 May 2010 when the Dow Jones on Wall Street lost almost 9% of its value within minutes and hundreds of billions of dollars were wiped off share prices.

A spokesperson for Citigroup declined to comment.