Image copyright Thinkstock Image caption The investigation followed a tip-off that waste had been dumped off the coast of England

A US cruise liner is to pay a record fine of $40m (£31m) for dumping oil-contaminated waste, the US Department of Justice has said.

The investigation into Princess Cruise Lines followed a tip-off that the company had illegally discharged oily waste off the coast of England in 2013.

The department said the firm had also tried to cover up its waste dumping.

It has agreed to plead guilty to seven charges related to stops at ports in nine US states and two territories.

The charges relate to the Caribbean Princess cruise ship, which the department said had been making illegal discharges since 2005, one year after the vessel started operations.

This was done using equipment - including a so-called "magic pipe" - to bypass pollution-prevention tools that separate oil and monitor oil levels in the ship's water, the department said.

'Clear message'

US investigators began to probe the ship's actions after an engineer reported an illegal dump off the coast of England in August 2013.

The engineer quit his position when the ship reached Southampton.

The ports linked to the charges were in Florida, Maine, Massachusetts, New Jersey, New York, Rhode Island, South Carolina, Texas and Virginia, as well as the U.S. Virgin Islands and Puerto Rico, the department said.

Jeremy Smart, head of enforcement at the Maritime and Coastguard Agency, said the case sent "a clear message to the industry that this kind of pollution practice will not be tolerated anywhere in the world".

Princess Cruise Lines is a subsidiary of Carnival Corporation, the world's largest cruise company.

The US Justice Department said the fine was the largest-ever criminal penalty involving deliberate vessel pollution.