New rules to curb stock trading when markets plunge uncontrollably will go into effect in June for the largest stocks, sources familiar with regulators' plans said on Tuesday.

The plan was hastily crafted by the Securities and Exchange Commission and the major U.S. exchanges in response to the unexplained flash crash on May 6 that drove the Dow Jones industrial average down some 700 points within minutes.

The new restrictions known as circuit breakers will apply to all stocks in the Standard & Poors 500 index, one of the sources said, adding exchange-traded funds (ETFs) are excluded.

Regulators are still trying to pinpoint the cause of the market free-fall and are expected to disclose preliminary findings later on Tuesday.

Commodity Futures Trading Commission Chairman Gary Gensler called the report very preliminary.

There is still a great deal of work to do and a great deal of information to be reviewed, Gensler told reporters in Washington.

Regulators and the exchanges have been under intense pressure to zero in on what triggered the May 6 meltdown and to find ways to uphold the integrity of U.S. markets.

A circuit breaker or a mechanism to halt trading across markets and in a single stock has emerged as one of the key solutions to protect investors.

The breakers will act as speed bumps to help the market adjust quickly to the high levels of volatility, SEC Chairman Mary Schapiro said by video link from Washington at a conference in Boston.

The SEC and exchanges will propose later on Tuesday a circuit breaker that would halt trading in a stock for five minutes if it fell more than 10 percent in 5 minutes, multiple sources said.

One source said the breakers would apply between 9:45 a.m. and 3:35 p.m. EST, ending in time for the New York Stock Exchange's closing auction. The trial period will last six months ending in December.

The new curbs would align U.S. markets more closely to European markets, which have more muscular safeguards. Circuit breakers at the London Stock Exchange, for example, are based on the liquidity and volatility of individual stocks.

Regulators have been sifting through more than 17 million trades that occurred around the time the market went into free-fall. So far, they have discounted initial theories such as a computer hacker or terrorist activity. They have also dismissed the idea that a so-called fat finger triggered a large erroneous trade.

But no single cause has been articulated, now 12 days after the drop.

I'm a little skeptical of this solution, Chao Chen, a portfolio manager at TFS Capital, said of circuit breakers. The most important thing is to figure out what happened, because it seems to me that people are nervous in general about where the market is going.

Circuit breakers that would halt trading across all markets are also being considered. This would give investors time to digest any news and adjust trading strategies. There are already broad index-based breakers in place, but those were not tripped when the market shot down and then recouped some of its losses in less than 20 minutes that afternoon.

Regulators are mulling breakers that would temporarily stop trading when the broader market falls 5 percent, people familiar with the talks told Reuters. That is tighter than the minimum 10 percent threshold already in place.

(Reporting by Jonathan Spicer in New York, Rachelle Younglai and Christopher Doering in Washington, and Svea Herbst-Bayliss in Boston; editing by John Wallace, Matthew Lewis and Andre Grenon)