Britain's security has been threatened by 188 high-level cyber attacks in the last three months, according to a government security chief.
Ciaran Martin, chief executive of the National Cyber Security Centre (NCSC), told the Sunday Times many of the attacks "threatened national security".
Attempts by Russian and Chinese state-sponsored hackers were among those being investigated, he added.
Mr Martin spoke ahead of the official opening of the NCSC in London.
He told the newspaper that attempts on government departments were designed to "extract information on UK government policy on anything from energy to diplomacy to information on a particular sector".
These include alleged hacks similar to those on the US Democratic National Committee, which led to the publication of leaked emails from Hillary Clinton in the run-up to the US election.
US intelligence services have stated the attack was an attempt by the Kremlin to interfere in the presidential election.
Mr Martin said there had been "a step-change in Russian aggression in cyber space" over the last two years.
"Part of that step change has been a series of attacks on political institutions, political parties, parliamentary organisations and that's all very well evidenced by our international partners and widely accepted."
Meanwhile, Chancellor Phillip Hammond - a former defence and foreign secretary - said the NCSC had blocked 34,550 "potential attacks" on government departments and members of the public in the last six months - a rate of about 200 a day.
Writing in the Sunday Telegraph, he warned that the "internet revolution" raised the threat of being held to ransom by hackers, the theft of intellectual property and the "shutting down of critical national infrastructure".