The trial  run will begin after a 10-day comment period and will last through to 10 December the commission said. After the trial, the exchanges will re-evaluate the trading curbs and perhaps expand them to all stocks permanently.

In a separate report the SEC and Commodity Futures Trading Commission (CFTC) say they were unable to pinpoint the cause of the sharp market decline two weeks ago.

The agencies said the drop was caused by traders refusing to buy or sell, in both the stock and futures markets. There was also a heavy reliance by investors on automated orders to sell at the market price once stock prices had declined by a certain amount. And,there were different rules on different exchanges about when trading automatically slowed or stopped.

The agencies found no evidence that the market decline was caused by 'fat finger' trading errors, or by computer hacking or terrorist activity, but added “we cannot completely rule out these possibilities.”

The exchanges will continue to use different rules about slowing trading in some shares. The New York Stock Exchange (NYSE) said it elected to continue its system of “liquidity replenishment points,” which slows trading in stocks that move a certain amount in a certain time period. Those measures are used across the entire Big Board’s listings. The proposed individual circuit breakers apply only S.& P 500 stocks.

The SEC is to review the role of the so called self-help mechanism, under which one exchange can refuse to route orders to another if it perceives that orders are not being filled quickly enough.

That type of rerouting, around the most liquid markets and to markets where fewer investors were trading, was believed by the agencies to have exacerbated the decline on May 6. So, as trading slowed on the Big Board because of circuit breakers, orders routed around the exchange to smaller markets where prices quickly plunged.

During the six-month pilot period both SEC and CFTC will consider other measures, including ways to address the risks of market orders, a potential ban on 'stub quotes' of one or a few cents for a stock trading significantly higher than that, and study of the linkages between futures and equities markets, including the role of exchange-traded funds.