The line, which is owned by Carnival Corp, was fined £31 million after admitting it had polluted the sea, and tried to cover it up.

It came after 27-year-old Christopher Keays from Glasgow, who had been working as the third assistant engineer on the 3,192-passenger Caribbean Princess in 2013, found a “magic pipe” was being used to discharge thousands of gallons of waste into British waters, The Times reports.

Keays filmed and photographed the secret pipe as well as tanks which contained the oily bilge water, and onboard computers which had been manipulated to show false discharge readings.

When the ship docked in Southampton, Keays resigned and offered his findings to the Maritime and Coastguard Agency (MCA), which then presented it to the US coast guard who investigated the ship before it docked back in the US.

It was later discovered that the Caribbean Princess had been making the illegal discharges since 2005 – a practice that was also found on four other ships in the company’s fleet.

In addition to Keays’ reward, a quarter of the fine has also been earmarked for maritime conservation, with at least $1 million to be targeted at British projects.

In a letter to the US Justice Department, Keays, who studied mechanical engineering at the Glasgow College of Nautical Studies, said: “The disregard for lawful practice and pollution of the sea without remorse left me with a genuine sadness, that I was at the beginning of my career in an industry so irresponsible.”

In a statement, Princess Cruises said: “We are extremely disappointed about the inexcusable actions of our employees who violated our policies and environmental law.

“We are very sorry that this happened and have taken additional steps to ensure we meet or exceed all environmental requirements.”