The Friday letter from officials in the US, Britain and European countries comes after a State Department meeting where employees challenged Biden's policy.
The secretary of state was testifying before a Senate committee when pro-Palestinian demonstrators disrupted him.
Police clashed with demonstrators after parliament announced a new controversial law.
What does this say about how we police protests?
"Feudal mentality lives on."
As one person on Twitter said: "Is this decade just the 20th century on fast forward?"
The prospect of the monarch being confronted by protesters and piles of garbage in the streets of Paris forced officials to rethink their plans.
Only this week, it emerged that dozens of Iranian schoolgirls are being treated in hospitals for poisonings.
The anti-monarchy group Republic told HuffPost it plans to protest Charles and Camilla's upcoming appearance in York on April 6.
Wednesday was the largest coordinated day of industrial action for a decade.