The Foreign Office has since summoned the Russian ambassador and two members of the "Star Blizzard" group, which is believed to be controlled by the Centre 18 unit of the FSB, including one understood to be an intelligence officer in the Russian security service, have been sanctioned.

MPs, Lords, civil servants, journalists and others have been targeted in attempts to "meddle in British politics", Mr Docherty told MPs.

The group had "selectively leaked and amplified information" since 2015 using a technique known as "spear-phishing" to steal information from a "significant" number of parliamentarians from multiple political parties.

An official told Sky News: "Russia is targeting the UK's democratic process."

The group "acquires information for the Russian state. It is a group that supports FSB Centre 18.

"This information is used to undermine the West in various ways," they said. "This group has acquired a vast amount of data.

"It is very targeted - the number [of known hacks] is probably in the hundreds not thousands.

"We are coming towards an election year. We want to get this [hack and leak threat] more into the bloodstream - so people are more aware."

The attacks cited by the government include a 2018 hack on the Institute for Statecraft think tank and the leak of US-UK trade documents, which Jeremy Corbyn used in his 2019 General Election campaign.

Sir Richard Dearlove, the former head of MI6, was another apparent high-profile target.

He said he had been through "many more dramatic and worse things" than being hacked and "was not particularly concerned about it" but it "caused a huge amount of disruption".

"We are in a state of grey warfare with the Russians short of open aggression and conflict," he said.

"They will do anything to undermine critical infrastructure, national security and attack any of our institutions that are not pro-Russia