Chase has posted a message on its Web site apologising to customers who experienced problems connecting with the site on Wednesday. The intermittent outages provoked a storm of criticism on Twitter as the @chasesupport account closed down for the night with a note for customers to call a toll-free number for after-hours assistance.

The disruption to the Chase site comes a day after Bank of America appeared to suffer from a Distributed Denial of Service attack that slowed its online site to a crawl. BofA's problems emerged following threats from an Islamic group to attack its Web site and that of the New York Stock Exchange in retaliation for the release of a US movie mocking the prophet Mohammed.

The bank-backed Financial Services Information Sharing and Analysis Cente (FS-ISAC) has raised its Cyber Threat Level from 'Elevated' to 'High' on the basis of "credible intelligence regarding the potential for DDoS and other cyber attacks against financial institutions".

The Web monitoring agency warns: "Members should maintain a heightened level of awareness, apply all appropriate updates and update AV and IDS/IPS signatures, and ensure constant diligence in monitoring and quick response to any malicious events."

Earlier this week the FBI warned of a new threat from cyber-criminals to disrupt bank Web sites as a diversionary tactic while attempting fraudulent wire transfers.

Neither JPMorgan Chase nor Bank of America have offered an explanation for the cause of the problems on their Websites.