For a second time, Carnival Corp's Princess Cruises has pleaded guilty in a US federal court to violating the terms of probation that was part of the shipowner's conviction for environmental crimes.

The US Justice Department said that under its new plea agreement, the cruise line will pay an additional $1m and to make sure it takes steps to create an independent investigative office.

As part of the plea deal, Princess and Carnival admitted to failing to set up that office, which had been required under the probation.

The deal comes after US officials sought in November to revoke probation after what were described as repeated findings that Carnival's internal investigation programme is inadequate.

A third-party auditor and a court-appointed monitor said in October that the problem "reflects a deeper barrier: a culture that seeks to minimise or avoid information that is negative, uncomfortable, or threatening to the company, including to top leadership", according to the Justice Department.

However, since that time, Carnival has made changes. Independent auditing that had previously reported to a single executive now reports directly to the Carnival board of directors.

The latest plea agreement comes nearly five years after Princess was convicted of dumping oil-contaminated waste and taking measures to cover it up. After pleading guilty, the cruise line was sentenced to pay $40m — the largest-ever criminal fine for pollution from ships — and serve five years of probation.

Its probation had impacts across the fleet of Carnival, the world's biggest cruise ship owner. The company's ships that called in US ports had to comply with an environmental compliance programme.

Then in 2019, Princess was fined another $20m after it was convicted of violating that probation.

"As the court has acknowledged, we have been working in good faith through the course of probation and have already implemented a number of the changes in the order, even prior to the order, to improve in the area which is the basis of the proceedings — our internal investigations," Carnival said in a statement provided to TradeWinds.

"We will continue to honour our highest commitment to strive for excellence in compliance, environmental protection and the health, safety and well-being of our guests, the people in the communities we touch and serve in which we operate and our shipboard and shoreside personnel."