The London Stock Exchange said its trading system in fully operational again after over four hours of problems.
From about 9.30 this morning 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 stock exchange said the problems had nothing to do with the core trading platform that it is replacing, known as Tradelect.
"Since 10.33 customers have been able to put orders on and take them off but are not executing," said a spokesman.
He said the issue has not yet been resolved.
In September last year the London stock exchange was offline for about eight hours as a result of technical problems.