The LSE says it was experiencing connectivity issues
Trading on the London Stock Exchange (LSE) was halted for three and a half hours earlier because of technical difficulties.
The LSE said it had been affected by connectivity issues, and at 1033 GMT had placed all orders for shares into an "auction call period".
This allowed traders to put orders to buy or sell shares into the system, ready for when trading restarted.
Normal trading was then able to resume from 1400 GMT.
"We regret the inconvenience that today's disruption to trading has caused for our clients," said London Stock Exchange chief executive Xavier Rolet.
"Having resolved the immediate issue, we are working hard to ensure this doesn't happen again ahead of switching to [its new] Millennium IT's trading platform next year."
The FTSE 100 index was frozen at 5,264.97 when trading was interrupted.
After trading was resumed it went on to finish the day at 5,194, which was 3.2% lower than Wednesday's closing price.
This fall mirrored similar declines across Europe, due to concerns about the wider impact of state-owned investment company Dubai World asking for a six-month delay on repaying its debts.