The London Stock Exchange cannot complete trades as the result of connectivity problems.
The exchange realised it was having problems at about 9.30am after identifying that some of its customers could not connect properly.
The stock exchange said it is still having problems but they have nothing to do with its core trading platform, Tradelect, which it is replacing because it is not fast enough in a highly competitive market.
"Since 10.33am 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.