Bosses of Britain's defence firms which supply weapons to Ukraine have been warned they are at risk of being targeted by Russian spies, The Mail on Sunday has been told.

The chiefs were notified about the elevated threats they and their staff face from the Kremlin during a roundtable between senior government officials and CEOs earlier in the summer. 

A source did not confirm which specific bosses were present, but they are thought to be those of businesses supplying weaponry and drones to Ukraine.

A number of defence companies have already come to prominence because of their equipment sales. 

They include Ukrspecsystems, based near RAF Mildenhall in West Suffolk, which makes drones for Ukraine, and BAE Systems subsidiary Callen-Lenz – the supplier of those used recently to strike oil refineries deep inside Russia.

Also prominent is MBDA, which makes the Storm Shadow missiles used by Ukraine to attack Vladimir Putin's country.

The Foreign, Commonwealth & Development Office meeting came after the bosses of two German defence companies became targets of foiled Russian assassination plots.

One of them was Armin Papperger, the CEO of defence giant Rheinmetall, which had a £4.2billion contract to supply artillery shells to Ukrainian forces.

Elisabeth Braw, a security expert at the Atlantic Council think-tank, said: 'Leaders of defence companies in the UK and in Europe are beginning to think about their own security and the security of their companies' installations.

'Russia is signalling that everybody who is involved in the defence enterprise is now considered a legitimate target for the Russians.'