A vociferous environmental campaigner, the Prince of Wales told government, corporate and non-profit leaders at a hotel in the US capital yesterday: "One issue that we absolutely cannot ignore is that of the increasing quantity of plastic waste in the marine environment.
"I was horrified to learn that, according to recent research, we collectively allow as much as eight million tonnes of plastic to enter the oceans every year."
Earlier, in lighter scenes, the royal couple mingled with gobsmacked young Americans as they hit some of Washington's top tourist attractions.
Bright sunlight but chilly temperatures prevailed as they inspected the Lincoln Memorial and Martin Luther King monument.
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Later they ventured out to Mount Vernon to look around George Washington's patrician home, which commands a grand view over the lower Potomac.
Prince Charles, 66, also took time to inspect the National Archives' copy of the Magna Carta, signed eight centuries ago this year by his predecessor, England's King John.
It is the 20th official visit to the United States for Charles, who is assuming more and more of the duties once undertaken by his elderly mother.