'Hostile states' are said to be responsible for the bulk of thwarted strikes (Picture: MOD).
The UK will inevitably be hit by the most serious type of cyber emergency, according to the head of the National Cyber Security Centre.
It has emerged that a specialist unit is repelling around 10 attempted attacks every week.
Ciaran Martin said he had 'little doubt' it would have to deal with a 'category 1' case in the future.
The bracket covers incidents that result in severe economic or social consequences, or even loss of life.
The NCSC is publishing a report on Tuesday that details the scale of the danger.
Mr Martin, the NCSC's chief executive, said: "The majority of these incidents were, we believe, perpetrated from within nation states in some way hostile to the UK.
"They were undertaken by groups of computer hackers directed, sponsored or tolerated by the governments of those countries." He added:
"These groups constitute the most acute and direct cyber threat to our national security.
"I remain in little doubt we will be tested to the full, as a centre, and as a nation, by a major incident at some point in the years ahead."
The NCSC defines a category 1 incident as a national cyber emergency, which causes "sustained disruption" of essential services or affects national security, leading to severe economic or social consequences or to loss of life.
Although there have been several very significant incidents, Mr Martin said the UK has so far avoided a category 1 event.
The assessment by the NCSC comes less than a fortnight after Britain accused the GRU, the Russian military intelligence service, of being behind a campaign of cyber attacks targeting political institutions, businesses, media and sport.
Cyber defence is an issue at the heart of the Government's overall defence aims.
Last month, the Defence Secretary Gavin Williamson announced plans for a new generation of cyber cadets.
The new scheme is to help develop the next generation of cybersecurity experts to deal with and thwart evolving threats.