The Oscar-winning actor said British journalists had twisted his comments the day before in support of Argentina's push for a U.N.-sponsored negotiated settlement to the sovereignty dispute.
"Good journalism saves the world. Bad journalism destroys it," Penn said.
London's conservative Daily Mail called the statement "an ugly attack on the press" by a "left-wing U.S. actor" in its Wednesday editions and quoted a member of parliament, Patrick Mercer, as calling Penn's comment "moronic."
"My oh my, aren't people sensitive to the word 'colonialism,' particularly those who implement colonialism," Penn said Tuesday night. He then doubled down on his criticism of Britain's actions leading up to the 30th anniversary of Argentina's failed 1982 invasion of the Falkland Islands, a British territory that Argentines claim as the Malvinas. More than 900 people were killed in the war.
Britain says it will never negotiate with Argentina as long as the Falklanders want to be British. It has sent Prince William on a six-week air force mission to the islands, along with its most powerful destroyer, the HMS Dauntless. The British government also hasn't denied reports that it sent a nuclear submarine, possibly with nuclear missiles, to the disputed southern seas.
"It's unthinkable that the United Kingdom can make a conscious decision to deploy a prince within the military to the Malvinas, knowing the great emotional sensitivity both of mothers and fathers in the United Kingdom and in Argentina who lost sons and daughters in a war over islands with a population of so few," Penn said.
Britain denies it is militarizing the dispute, and notes that William is serving simply as a search-and-rescue helicopter pilot.
Penn called it a provocation nonetheless.
"There are many places to deploy a prince," he said. "It's not necessary when the deployment of a prince is generally accompanied by a warship, to send them into seas of such spilled blood."
Penn, who runs a Haiti relief organization and was named "ambassador at large" by Haiti's government, spoke after meeting with President Jose Mujica of Uruguay, which contributes to the U.N. peacekeeping force in Haiti.
The Daily Mail quoted Mercer, a conservative military veteran, as saying Penn should stay out of the Falklands dispute.
"What on earth has this got to do with Sean Penn? He's neither British nor Argentine and seems to know nothing about the situation," Mercer said. "A good number of his movies have been turkeys, so I suppose we shouldn't expect much better coming out of his mouth."
Based on reporting by the Associated Press.