Russia’s foreign intelligence service is trying to set up a new spy network in Britain after the military unit behind the Salisbury nerve agent attack was dismantled in the UK, according to well placed sources.

Authorities are increasingly concerned by attempts by the SVR, Russia’s foreign intelligence agency, to re-establish a foothold in Britain. Officials are confident that the GRU, the agency responsible for the attempted assassination of Colonel Sergei Skripal, has been effectively neutralised.

That follows a detailed counter-terrorism and intelligence-led investigation into the use of weapons grade, Novichok nerve agent that exposed the GRU’s network of agents in the UK and across Europe.

But it is now understood that the SVR - equivalent to Britain’s MI6 - has been tasked by the Kremlin with resuming operations in Britain.

The view among British intelligence officials is the SVR is a more professional and dangerous outfit than the GRU and poses a bigger threat.

A senior Government source said: “We are more fearful of what we don’t know about the SVR compared to all the things we do know about the GRU.

"If Moscow is now giving more resources to the SVR and more freedom to operate in the UK - which is what we believe is happening - than that is of far greater concern because they are a more professional outfit.”