A British warship accidentally launched an inert torpedo during a training exercise, sending a dummy weapon crashing into a wharf at a naval base in southern England used by nuclear submarines.

The Ministry of Defence (MoD) said an investigation was under way into the incident, which occurred at Devonport naval base in the city of Plymouth on Britain's south coast, the largest such facility in Western Europe.

"We can confirm an incident occurred onboard HMS Argyll, while alongside at Devonport Naval Base," an MoD spokesman said. "During a training exercise, an inert Test Variant Torpedo unexpectedly jettisoned onto the wharf. There was no explosion and no casualties."

Devonport, a sprawling 2.6 million sq metre site, is used by Britain's nuclear submarine fleet as well as hosting training facilities and the country's assault fleet. The MoD did not comment when asked whether any nuclear submarines were present at the time of the incident.

HMS Argyll is the longest-serving type-23 frigate in the British navy and was refitted in 2009/10. The ship, which last year conducted anti drug-smuggling operations in the eastern Pacific, is described on a government website as "one of the most up-to-date and capable frigates in the fleet".