Feb 23 (Reuters) - Shares of International Business Machines IBM.N recorded their steepest daily drop in more than 25 years on Monday, after AI startup Anthropic said its Claude Code tool could be used to modernize a programming language run on IBM systems.

IBM shares sank 13.2%, their biggest drop since October 18, 2000.

COBOL is a programming language widely used on IBM mainframes across banking, insurance and government systems.

"Modernizing a COBOL system once required armies of consultants spending years mapping workflows. Tools like Claude Code can automate the exploration and analysis phases that consume most of the effort in COBOL modernization," Anthropic said in a blog post on Monday.

"With AI, teams can modernize their COBOL codebase in quarters instead of years," it added.

Software stocks have been battered in recent months by market fears around the growing capabilities of AI tools, particularly following the launch of plug-ins from Anthropic's large language model Claude, seen as the startup's push to become an application layer.

Shares of cybersecurity companies including CrowdStrike CRWD.O and Datadog DDOG.O also slumped on Monday, as investors weighed the potential impact of Anthropic's new security tool on the industry.

(Reporting by Chris Thomas in Mexico City; Editing by Janane Venkatraman)

((chris.thomas@thomsonreuters.com;))