LONDON (AP) — The London Stock Exchange Group PLC said yesterday that a software glitch — and not high trading volumes — was to blame for a seven-hour shutdown that angered customers on one of the busiest days of the year on world equity markets.

The LSE, which is facing increasing competition from new European trading platforms, also attempted to reassure traders unhappy about the blackout, saying that it had changed the software programming so that a similar problem would not have such a huge effect.

The lengthy shutdown wiped out most of Monday's trading session, leaving many clients unable to cash in on a worldwide stock market boom that followed the US government bailout of mortgage giants Fannie Mae and Freddie Mac.

The LSE's confirmation of a software issue discounted speculation from traders that the high volume of business in the hour of trading — the leading FTSE 100 index surged almost four percent — before the system seized up could have been a factor.

"It was related to software and two computer events that coincided," the exchange said. "We have put in a fix and we are confident that should the coincidence recur, it won't have the same impact."

The market was trading as normal yesterday, with the FTSE 100 index up 0.5 percent mid-afternoon, but many customers warned that the biggest disruption to trading in more than eight years would likely speed business to the rival platforms.