A Russian warship was caught opening fire just miles from the Devon coast, with the Royal Navy powerless to intervene.
One of Vladimir Putin’s frigates, the Neustrashimy, was seen launching a live-fire artillery exercise around 40 nautical miles south of Plymouth.
The Royal Navy monitored the firing and is now tracking the vessel’s activity closely, a Ministry of Defence spokesman said.
French military aircraft also monitored the Russian warship and asked it to clarify its intentions by radio.
The Neustrashimy told the Royal Navy Offshore Patrol Vessel HMS Tyne it intended to conduct the exercise.
It then requested the HMS Tyne to move back to a safer distance, which it did.
The live-firing exercise took place for 30 minutes in international waters.
Last week, the Neustrashimy, part of Russia’s Baltic Fleet, was caught lurking in the North Sea near Felixstowe.
Russian Navy Frigate Neustrashimy conducted a live-firing exercise just 40 miles off Britain’s shores
Sky News cameras picked up the frigate as it was being refuelled and restocked by a Russian logistics support ship nearby.
UK defence sources would not comment to the BBC on whether Monday’s incident may have been timed to coincide with the start of Andy Burnham’s premiership.
But the incident is yet another barb from Russia and its “shadow fleet” in the waters just outside Britain’s control in the Channel and North Sea.
Last month, Royal Marine Commandos raided a “shadow fleet” oil tanker in the Channel in a six-hour operation.