The London Stock Exchange resumed trading just after mid-day, having resolved a "market data issue" that knocked out the order book for four hours on Friday morning.
The Exchange moved to halt trading in Sets and Setsqx at 07.54 and reverse or cancel trades affected by an apparent market data-related malfunction in the opening auction.
By 09.30, trading remained at a standstill, frustrating brokers who could only sit and watch as European markets rebounded and UK fourth quarter GDP data was released showing that the economy contracted even faster than previously estimated in the last three months of 2010.
With the LSE setting the benchmark price for stocks on the main market, trading remained thin on alternative venues such as Chi-X as traders stayed offline.
The LSE eventually reported a resolution of the problem at 11.37 and moved to delete all legacy orders from the order book.
By 11.44, the Exchange had placed the order driven markets into a 'resumption auction call' period with an uncrossing scheduled to take place at 12:15, and continuous trading expected to start immediately thereafter.
In a statement issued at 12.30, LSE chief Xavier Rolet apologised to market participants: "We sincerely regret the inconvenience that today's disruption to trading has caused our customers. We have resolved the real time data dissemination issue and our UK cash equity markets have now resumed trading. "
The breakdown is another embarrassment for the LSE, which moved its main markets to the MillenniumIT platform earlier this month. The switchover followed months of delays as the Exchange investigated technical problems which had hit trading on Turquoise following its migration to the new Linux-based system.
The migration of the cash markets has already been plagued by gremlins, as some bid/ask data delivered via terminals that use the FIX Protocol have failed to display correctly on broker screens. The Exchange is working with suppliers, including Thomson Reuters, SIX Telekurs and Interactive Data, to identify and iron out the problem.
Borsa Itlaiana, which is owned by the LSE and runs its old TradElect system, was also knocked out for six-and-a-half hours earlier this week due to a technical issue on DDMPlus, the real time data feed service used by the majority of domestic operators.