The Tokyo Stock Exchange suffered its worst ever outage yesterday when trading was suspended for four and a half hours due to a software problem.

A spokesman said that the glitch appeared to be connected to the decision to expand the trading system's capacity last month in response to high trading volumes.

The modified system had worked well, but crashed when the automatic monthly clean-up of the software was implemented. A back-up system also failed because it uses the same software.

The suspension was particularly embarrassing in light of current plans for a flotation of the exchange, although trading volumes were healthy in the remaining time available.