The London Stock Exchange failed to stop trading when scheduled today, just one day after completing a major system upgrade.
The glitch causing confusion among its clients, who blamed a technical problem.
The LSE, which ends normal trading at 16:30 before starting its closing auction, conceded on its website that normal trading did not cease until 42 seconds after the official close.
LSE clients were worried the slight delay might have meant that orders destined for the auction were mixed with normal trades, triggering concerns over which trades had been executed.
"Normal trading overran into what we thought was the auction so we didn't know what was going on for a while," said the head of electronic trading at one European broker.
The LSE, which said all trades would remain valid, added it was investigating the problem.
City A.M. Reporter