Share trading on the London Stock Exchange was halted at the open on Friday, infuriating traders, with the operator blaming a technical problem.
The latest incident came just two weeks after the LSE suffered a technical problem following the migration of a new trading system.
"There is a pause in order-driven securities trading, we are investigating problems in SETS and SETSqx," Lauren Crawley-Moore, spokeswoman at LSE said, adding that she was not able to say when the problem may be resolved.
SETS (Stock Exchange Electronic Trading Service) and SETS QX (Quotes and Crosses) are the LSE platforms on which traders place their orders.
"At a time of uncertainty in the markets, where traders are having to keep on their toes with the situation in Libya, the last thing they need is an unexpected halt to trading," Joshua Raymond market analyst at City Index said.
Britain's FTSE 100 had lost 2.7% in the previous five sessions on growing concerns that the unrest in Libya could spread to other oil-producing countries including Saudi Arabia, but on Friday European shares rebounded.
On Feb. 14, LSE launched its Millenium Exchange platform to offer investors faster execution and enable algorithmic trading.
The following day, LSE failed to stop trading at the close as scheduled causing confusion among clients.
LSE spokesman Alastair Fairbrother said it was not clear whether the new system was causing the problem on Friday.
"We can't say what's causing the problem or if it's to do with the implementation of or the migration to the new sytem," he said.
On Tuesday, Borsa Italiana, a unit of LSE, suspended its trading for hours because of a technical glitch.
The last serious trading halt was in November 2009 when the LSE stopped trading for three and a half hours.
"It's the international credibility that one has this slight worry about. London is supposed to be the leading financial centre of the world," said David Buik, senior partner at BGC Partners.
Equity traders were unable to react to data showing a surprisingly large drop in fourth quarter GDP, which prompted a fall in sterling and a rise in gilts, while there have been sharp moves in recent days caused by developments in North Africa.