"His fears about Russia getting pictures of power plants and studying the routes of British pipelines are worthy of a comic plot or a Monty Python's Flying Circus sketch", Russian Defense Ministry spokesman Igor Konashenkov said on Friday.
And Ciaran Martin, the UK's cyber-security chief, said it was a matter of "when, not if" Britain faced a major hacking attack in a bid to cause disruption to elections, in the wake of similar claims in the USA presidential vote in 2016.
He told Russian news agency Tass: 'Gavin Williamson in his fiery crusade for military budget money appears to have lost his grasp on reason.
Britain has three gas and four electricity interconnectors.
It will provide information to the quick reaction alert operation, which sees aircraft launched to police United Kingdom and global airspace - jets have been scrambled 69 times in the last five years.
In an interview with The Daily Telegraph, Gavin Williamson says Moscow has been researching and photographing power stations, and could take action that would leave millions of Britons without electricity.
He said it was "the real threat... the country is facing at the moment".
'Damage its economy, rip its infrastructure apart, actually cause thousands and thousands and thousands of deaths, but actually have an element of creating total chaos within the country'.
Mr Williamson, who became defence secretary last November, said Russian Federation had been researching these types of connections and would be willing to take action "any other nation would see as completely unacceptable".
"Our ability to pre-empt or respond to threats will be eroded if we don't keep up with our adversaries", he said.
His words echoed those of Britain's Chief of General Staff Sir Nick Carter earlier this week who claimed that the UK's armed forces could struggle to defeat Russian military prowess without a big increase in defence spending.
The warning comes as Mr Williamson aims to complete a defence review - called the Defence Modernisation Programme - by the summer.