A technical glitch delayed the start of trading on the UK blue chip FTSE 100 and midcap stock indexes for almost two hours.
It was the longest outage at one of the world's top bourses in eight years.
The London Stock Exchange suffered a "technical software issue", which postponed the opening of trading until 9.40am, a spokesperson said in an email.
The markets normally open at 8am.
The fault affected hundreds of shares in some of the UK's largest companies, including Shell, Unilever and HSBC and BHP Group, worth a total of $2.8 trillion.
Traders were frustrated by the latest outage coming during a hectic week on global financial markets, hit by worries about a US recession and the US-China trade spat.
This is the second outage at the LSE, which handles about £5 billion worth of volume each day, in just over a year.
The FTSE closed 0.7% higher this evening.
In June 2018, the opening auction was delayed due to a technical fault, but trading resumed after an hour.
Prior to that, the market was shut until lunchtime one day in February 2011.
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Thomson Reuters will own 15% in LSE after the proposed deal closes.
