UK government "badly underestimated" the Russian threat and the response it required, according to an inquiry.
The Intelligence and Security Committee's long-awaited report into Russian activity in the UK said the government was "playing catch-up" and needed to take "immediate action".
The report also claimed the government made no effort to investigate Russian interference in the EU referendum.
The Russian Foreign Ministry called the report "Russophobia".
No 10 said the government was "fully aware of the significant and enduring threat" Russia posed.
The ISC's inquiry covers a number of topics, including disinformation campaigns, cyber tactics and Russian expatriates in the UK.
Much of the "highly sensitive" detail was not published due to fears Russia could use the evidence to threaten the UK.