In messages seen by the BBC, Rob McCallum told OceanGate CEO Stockton Rush that he was potentially putting his clients at risk and urged him to stop using the sub until it had been classified by an independent body.

Mr Rush responded that he was “tired of industry players who try to use a safety argument to stop innovation”.

The tense exchange ended after OceanGate’s lawyers threatened legal action, Mr McCallum said.

“I think you are potentially placing yourself and your clients in a dangerous dynamic,” he wrote to the OceanGate boss in March 2018. “In your race to [the] Titanic you are mirroring that famous catch cry: ‘She is unsinkable'”.

[Hoo boy. Passengers signed waivers on liability, but those are not an absolute shield against lawsuits. OceanGate’s remaining officials and investors may wind up losing a significant amount of money in court at some point, especially given the cavalier dismissal of what turned out to be accurate warnings. — Ed]