The London Stock Exchange's trading platform Tradelect is offline following technical problems.
The glitch comes on what was expected to be a busy day following the US government's decision to bail out mortgage lenders Fannie Mae and Freddie Mac.
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The stock exchange realised it had a problem at 9.15am this morning and has been working since then to identify and fix the problem.
A source close to the company said an upgrade had gone wrong. The stock exchange would not comment.
A spokesman at the stock exchange said the system was put in auction mode at 11.45am, which means investment firms can place and delete orders but no automatic execution takes place.
"That's where we are now," said the spokesman at 2.15pm. "We are still working on the problem and we will let people know when we move out of auction mode."
Chris Skinner, CEO at financial services think tank Balatro, said the downtime is particularly damaging because the market is very busy following the US government's decision to bail out Fannie Mae and Freddie Mac.
"This would have been one of the biggest trading days this year and there will be a lot of business going to competitors," he said.
The London Stock Exchange has recently announced a cut in the fees it charges clients as well as faster trading services to help fend off increased competition.
"At the moment the London Stock Exchange is trying to fight its competitors off and seems to be losing the battle," added Skinner.