The London Stock Exchange says its trading system is fully operational again after more than four hours of problems.
From about 9.30am the exchange could not complete trades as the result of connectivity problems.
"The system is has been fully up and running from 2.00pm," said a spokesman.
He said the company is still looking into the cause of the problems.
The London Stock Exchange said the problems had nothing to do with the core trading platform that it is replacing, known as Tradelect.
In September last year the London stock exchange was offline for about eight hours as a result of technical problems.