Cyber attacks on Government computer systems and vital parts of the economy have become one of the greatest threats to national security, an official report disclosed yesterday.

The Government confirmed for the first time that a series of electronic assaults on official and private sector databases have already been carried out and are continuing.

Though the Cabinet Office report did not identify who carried out the 'well-resourced and sophisticated attacks', Whitehall sources disclosed that China is suspected of state-sponsored espionage.

Hacking: China has furiously denied allegations it wants to sabotage the world electronically

Beijing has issued furious denials that it was engaged in such practices and the timing of yesterday's warning is particularly sensitive, coming on the opening day of the Olympics.

But intelligence officials fear there is clear evidence China is mounting an aggressive push to establish 'electronic dominance' over its global rivals.

It would use cyber-warfare to cripple an enemy's financial, military and communications capabilities in a conflict.

Chinese groups are also said to be using 'Trojan' software to hack into the networks of firms and Government departments and access confidential data.

Jonathan Evans, director-general of MI5, has sent a letter to 300 chief executives and security chiefs at banks, accountants and legal firms warning them they are under attack from 'Chinese state organisations'.

Other countries - notably Russia - are thought to be using similar techniques.

Such cyber-attacks have been identified as one of the two most likely threats to the UK, the other being a terrorist attack on the transport network.

Cabinet Minister Ed Miliband published the first 'national risk register' which disclosed previously secret assessments of the dangers posed to Britain by foreign states, terror groups, natural disasters and major accidents.

The document, which will be updated annually, is designed to enable communities to prepare for potential disasters.

It revealed there was a 'known risk to commercially valuable and confidential information in some Government and private sector systems from a range of well-resourced and sophisticated attacks'.

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The report added: 'IT systems in Government departments and various organisations, including elements of the national infrastructure have been and continue to be attacked to obtain the sensitive information they hold.

'Some of these attacks are well planned and well executed.'

As the Daily Mail revealed yesterday, the register identified a flu pandemic as the deadliest potential threat to Britain, though it is considered less likely to occur than cyberattacks and terrorist incidents.

The report warned half the population would be affected by an outbreak, which could kill between 50,000 and 750,000.

It said the Government has stockpiled enough doses of the antiviral drug oseltamivir, known as Tamiflu, to treat a quarter of the population. This should be enough to combat a pandemic the size of those in the 20th century, it said.

The report urged families to take a series of precautions to prepare for a major incident by stockpiling bottled food, water, a battery-run radio, a torch and having back-up heating.

They should prepare backups or print-outs of key documents stored on a computer, make alternative arrangements for vital journeys, such as getting to work or school, if public transport is not working.

Security Minister Lord West rejected suggestions that publication of the register would cause unnecessary alarm.

'I have great faith in the public and they would prefer to have information because the thing that causes fear is ignorance of things,' he said.

Shadow security minister Baroness Pauline Neville-Jones welcomed the publication of this register, but warned: 'We compare extremely poorly with comparable advanced economies when it comes to stockpiling oil and gas resources to sustain us through an emergency.'