The virus, which knocked out trading on all of its three markets between 1315 and 1420 GMT on Thursday, was a deliberate attack, RTS said.

Once the offending program had been traced to one computer, which was then turned off, there were no irreversible effects, it said.

"The virus got into a computer connected to a test trading system from the Internet," RTS Vice President Dmitry Shatsky said in the statement.

"The infected computer started generating huge volumes of parasitic traffic, which overloaded the RTS's support routers. The result was that normal traffic -- data going into and out of the trading system -- was not processed."

The virus struck the exchange the day before a destructive e-mail was expected to attack computers worldwide. Security experts have warned that the "Kama Sutra" worm is timed to attack on February 3, freezing computers and deleting files.