Trading in the biggest shares listed on the London Stock Exchange was delayed for more than an hour, in the longest outage in more than eight years.
Shares in the FTSE 100 and FTSE 250 indexes were affected, although smaller stocks traded as normal at 08:00.
Trading resumed at 09:40 - the longest closure since February 2011, when it was shut for more than four hours.
The last time the market opened late was in June 2018, when it was delayed by an hour.
The LSE said trading was delayed while it investigated a "potential trading services issue".
When trading eventually began, the FTSE 100 - which had fallen to a six-month low on Thursday - rose, and by midday it was 43.99 points higher at 7,111.
The London Stock Exchange said it had "experienced a technical software issue this morning that affected trading in certain securities, including FTSE 100 and FTSE 250 stocks".
"Following resolution of the issue regular trading in all securities commenced at 09:40," a spokesperson for the exchange said.