Journalists said they should be allowed to cover a legitimate news story, but Israel said the media would be complicit in an illegal breach of its naval blockade of a hostile territory ruled by a terrorist group.
The announcement reflected Israeli jitters about the international flotilla, which comes just more than a year after a similar mission ended with the deaths of nine Turkish activists in clashes with Israeli naval commandos who intercepted them. Each side blamed the other for the violence.
Israel is eager to avoid a repeat of last year's raid, which drew heavy international condemnation and prompted Israel to ease its blockade on the Hamas-controlled Gaza Strip.
Many Israelis believe that the media's coverage of the bloodshed contributed to their country's image problems.
In a letter to foreign journalists, the Government Press Office's director, Oren Helman, called the flotilla "a dangerous provocation that is being organized by western and Islamic extremist elements to aid Hamas."
He warned journalists that taking part in the flotilla "is an intentional violation of Israeli law and is liable to lead to participants being denied entry into the State of Israel for 10 years, to the impoundment of their equipment and to additional sanctions," Helman said.
The letter, he added, was reviewed and approved by Israel's attorney general.
The Foreign Press Association, which represents hundreds of journalists working for international news organizations in Israel and the Palestinian territories, condemned the Israeli decision and urged the government to cancel the order.
"The government's threat to punish journalists covering the Gaza flotilla sends a chilling message to the international media and raises serious questions about Israel's commitment to freedom of the press," the FPA said in a statement.
The association's lawyer, Gilead Sher, sent a letter to Helman demanding that the threat be rescinded.
"The state of Israel must make a clear distinction between those who carry out an action and those who cover it (as journalists)," Sher said in his note. Israel's local journalists association also condemned the government's letter.
Israel sees the flotilla as a provocation aimed at stirring up trouble and says that it has standard channels for delivering humanitarian aid to Gaza.
Israel imposed its land and naval embargo on Gaza after Hamas, an Iranian-backed group that rejects peace with Israel, overran the territory in 2007. Under international pressure, Israel eased the land blockade after last year's flotilla raid, but the naval closure remains intact.
Israel says the blockade is justified on internationally accepted legal grounds because it is effectively in a state of war with Hamas.
Critics say that while Israel is entitled to stop arms smuggling, the blockade has amounted to collective punishment and caused widespread suffering among Gaza's 1.6 million people.
The pro-Palestinian organizers of the flotilla say the mission is necessary to draw attention to the Gazans' plight.
It remains unclear when the current flotilla will actually set sail, but organizers have hinted it could be as soon as this week.
Organizers have said 10 boats, including two cargo vessels carrying aid supplies, will participate in the flotilla and that hundreds of people, including activists, journalists, politicians, writers and religious figures, will be on board.
About two dozen activist groups, many of them based in Europe, are organizing the journey. Among them are İHH, a Turkish Islamic charity that helped organize last year's flotilla and is outlawed in Israel.
There are already growing signs that this flotilla will be different.
The Turkish boat that clashed with the Israeli navy has dropped out of this year's flotilla. After a year of tensions, both Turkey and Israel have signaled that they are interested in repairing strained ties.
Greece has also urged Greek citizens and Greek-registered vessels not to participate, noting the risk of violence. Activists on "The Audicity of Hope," an American boat in the flotilla that plans to sail from a Greek port, said police declared they could not leave because of questions about the vessel's seaworthiness. In a statement, the activists speculated the delay stemmed from Israeli and US pressure.
Manuel Tapial, a Spanish activist, said a Spanish vessel in the flotilla planned to depart for Gaza this week in coordination with other aid boats and would be carrying 11 journalists, most of them Spanish. He criticized Israel's warning to reporters, saying Israel did not want witnesses "to what they can do to us on the water."