A US judge has allowed a case to proceed against UK traders accused of causing an unprecedented crash in oil futures markets after ruling that text messages between the group were sufficient to point towards a potential conspiracy.

The proposed class-action lawsuit centres around trading on April 20 2020, when the price of benchmark West Texas Intermediate crude oil plunged below zero for the first time on record. Self-employed traders linked to Vega Capital London, a little-known commodities brokerage based in the Essex town of Benfleet, stand accused of making more than $700mn by flooding futures markets to drive the price lower.

Mish International Monetary, a California rare coin dealer that says it lost money that day, alleges market manipulation and violations of antitrust laws. Its civil suit accuses the defendants of buying special “trading at settlement” (TAS) contracts, whose price would be fixed by that day’s settlement value, then conspiring to dump huge volumes of ordinary WTI contracts ahead of the expiry.

Court documents released on Tuesday and first reported by Bloomberg show how the group — dubbed the Essex Boys — discussed trading strategies that day over WhatsApp.

“I’m doing a fortune on TAS haha,” one of the traders is quoted as saying.

“We pushed each other so hard for years for this one moment . . . And we f****** blitzed it boys,” said another. A third added: “Please don’t tell anyone what happened today lads.”

US District Judge Gary Feinerman of Chicago gave the go-ahead for the case to progress, ruling that the allegations against eight of the 12 traders accused were plausible based on the communications, as well as on a “high degree of correlative trading”.

The defendants, whose identities are protected by court order, deny the allegations and contend they are independent traders who were following market signals during a period of turmoil. Vega Capital could not be reached for comment.

WTI futures on CME Group’s New York Mercantile Exchange had crashed by $56 a barrel to close at negative $37.63 as a shortage of physical storage space led investors to dump futures contracts rather than take delivery. The price recovered to around $10 the next day, allegedly giving the group its windfall.

In the period immediately before WTI futures expired, the collective was dumping 153.5 contracts a minute and accounting for more than 30 per cent of total global market volume, the Mish lawsuit alleges. It notes an “extremely strong tendency” of between 96.2 per cent and 99.7 per cent for its trades to “move in the same direction at the very same time”.

The Commodity Futures Trading Commission, the US regulator, analysed the WTI price plunge on April 20 2020 but it did not draw conclusions as to the cause.

Feinerman dismissed the case against Vega Capital and its owner, ruling that they were not party to the alleged conspiracy. Mish has until April 28 to challenge the judge’s decisions by amending its complaint.