Charles Schwab, the largest online brokerage, experienced a 90-minute system-wide breakdown on Wednesday, denying its customers access to internet-related services including the ability to trade through the internet.

The company admitted its technology was nearing the limits of its capacity, despite a massive effort to upgrade its systems.

Schwab's is the latest difficulty to hit large online brokers, which continue to struggle with rising demand and trading volumes. Earlier this month the third largest broker, E*Trade, suffered a three-day trading "outage".

The system problems at Schwab appeared to be more serious, since they brought to a halt not just trading, but all access to other online services such as portfolios and research reports.

The company blamed one of its mainframes for Wednesday's outage.