RUSSIA has hacked Britain’s energy grid in a devastating suspected bid to one day blackout Britain.

The senior spy will also reveal Russian agents tried to penetrate major British telecommunications companies, such as BT.

And they have successfully targeted media organisations too – at times even bringing down their websites with denial of service attacks.

Mr Martin’s bombshell revelations lift the lid on what amounts to the most serious threat to Britain from Russia since the end of the Cold War 26 years ago.

They follow PM Theresa May’s warning to Russia boss Vladimir Putin on Monday night over cyber hacking, when she told him: “We know what you are doing”.

The power grid hack this summer triggered alarms, and experts from eavesdropping spy service GCHQ were called in to flush out ‘sleeper’ worms.

It is believed the Russian state-sponsored hackers intended to lie dormant inside the energy network after having penetrated it, to then cause significant damage at a later date.

Speaking at The Times Tech Summit tomorrow, Ciaran Martin will say: “I can’t get into precise details of intelligence matters.

“But I can confirm that Russian interference, seen by the National Cyber Security Centre over the past year, has included attacks on the UK media, telecommunication and energy sectors.”

He will add: “Russia is seeking to undermine the international system. That much is clear.

“The PM made the point on Monday night – international order as we know it is in danger of being eroded.”

The NCSC – an off shoot of GCHQ – also today revealed for the first time that it has had to deal with more than 600 cyber attacks since it was created just over a year ago in October 2016, or almost two a day.

Of those, 35 were designated “significant incidents”, where the threat to the country was deemed serious enough for government ministers and spy agencies’ full resources to be called in.

Ireland’s Electricity Supply Board was also hit by a cyber hack in June, for which Russia’s GRU intelligence agency was blamed.

Asked whether Mrs May was worried about Russia meddling in UK elections in the wake of her speech on Monday night, the PM’s spokesman said: “Our electoral system is amongst the most robust in the world.

“But obviously we continue to make sure we take all possible steps to protect the integrity of our electoral system.”

“We haven’t seen any successful direct attempts to meddle in the UK electoral systems.”