* Confidentiality drilled into clerks "from day one"

* No leaks of Supreme Court rulings in recent decades

By James Vicini

WASHINGTON, March 30 (Reuters) - U.S. Supreme Court justices on Friday held closed-door deliberations on President Barack Obama's healthcare overhaul law, likely casting preliminary votes on how they will eventually rule on their highest-profile case in years.

In an institution known for keeping its secrets, no leaks are likely before formal opinions have been written and announced from the bench. That is not expected to occur until late June, when the court is set to go on its regular summer recess.

The justices' private conference, a meeting in which they typically discuss and vote on cases heard earlier in the week, came after three days of historic arguments over the healthcare law that ended on Wednesday.

Legal experts said only a handful of people - mainly consisting of the nine justices and their law clerks - know about the outcomes of these conferences, and they do not talk about it. Law clerks are sworn to secrecy.

"Confidentiality is drilled into clerks from day one," said University of Richmond associate law professor Kevin Walsh, a law clerk to Justice Antonin Scalia in the court's 2003-04 term.

"The rules and warnings only heightened the obligation we already felt to maintain confidentiality born out of our respect for the Supreme Court and our desire to protect it," he said.

"And it's not like working for the CIA, where you may take secrets to the grave. The big news of any given term - what the court has decided - all comes out into the open by the end of June," Walsh said.

The Supreme Court's private conferences are held with only the justices attending. The meeting room, located on the second floor, is relatively small, oak-paneled and with a fireplace and a rectangular table. It is just off the chambers of Chief Justice John Roberts.

"WE SHOULD REPORT IT"

In recent decades there have been no leaks of Supreme Court rulings, including the momentous 2000 decision that stopped a Florida vote recount, clearing the way for Republican George W. Bush to become president over Democrat Al Gore.

There have been no leaks in high-stakes financial cases including ones affecting the tobacco industry. Stocks of insurers and other healthcare companies could be roiled by any ruling on the two-year-old healthcare law, Obama's signature domestic policy achievement.

The last time Supreme Court leaks emerged as an issue was under Chief Justice Warren Burger, who left the court in 1986.

Then-ABC TV journalist Tim O'Brien reported in 1986 that the court the next day would strike down a key part of a law to balance the U.S. government's budget. He was right about the outcome, but the ruling did not come down until weeks later.

In 1979 he correctly reported the ruling in a major libel case involving the CBS News television show "60 Minutes."

Burger accused an employee in the printing shop of tipping O'Brien and had the employee transferred. The employee denied disclosing any information about the ruling.

"The court has the right to protect its secrets," said O'Brien, who has left ABC and who acknowledged that leaks of rulings are rare.

"But if the news media learns about it, we should report it," said O'Brien, an attorney who has taught law. "People don't watch us or read us because of our ability to keep the government's secrets."

In 1973 Time magazine correctly predicted the court's historic decision that women have a constitutional right to an abortion. Burger then warned all the law clerks not to speak to or be seen with news reporters.

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