MI5 chief Ken McCallum said in a speech on Tuesday that the GRU was "on a sustained mission to generate mayhem on British and European streets: we've seen arson, sabotage and more," Reuters reported.
Kremlin spokesman Dmitry Peskov told reporters that Russia would not accept such allegations and they were not worthy of attention.
"All these statements are absolutely unsubstantiated and unfounded," he said.
Against the background of the war in Ukraine, European countries and the NATO alliance have accused Russia of mounting a range of hostile activities across the continent in recent months.
In a joint article for the Financial Times last month, the heads of the US Central Intelligence Agency and Britain's foreign spy service MI6 said they were working together to disrupt a "reckless campaign of sabotage across Europe being waged by Russian intelligence, and its cynical use of technology to spread lies and disinformation designed to drive wedges between us".