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The UK government "actively avoided" looking for Russian interference during the EU and Scottish independence referendums, says a group of MPs.
The Intelligence and Security Committee's long-awaited report into Russian activity in the UK called for "immediate action" to tackle it.
But it said the government was "playing catch-up" after failing to act sooner.
Committee members also criticised No 10 for the delay in the report's publication.
Downing Street was accused of holding back the committee's report ahead of December's UK election - which it has denied.
But speaking at a press conference to launch the report, one of the ISC's committee members, Kevan Jones, criticised the prime minister for not signing it off sooner, saying there was "no reason for delay".
After the report was published, Foreign Secretary Dominic Raab tweeted: "We've been clear that Russia must desist from its attacks on the UK and our allies.
"We will be resolute in defending our country, our democracy and our values from such Hostile State."
The ISC's inquiry covers a number of topics, including disinformation campaigns, cyber tactics and Russian expatriates in the UK - saying the UK was a "top target" for the state.
But much of the "highly sensitive" detail will not be published due to fears Russia could use the evidence to threaten the UK.
The report said Russian influence in the UK was now "the new normal", and said many governments had welcomed oligarchs from the country with "open arms".
The committee went on to say the UK was "clearly a target" for disinformation campaigns around its elections, but the issue was described as a "hot potato" - with no one organisation taking a lead to tackle it.
ISC committee member Stewart Hosie said no-one in Government wanted to touch the issue of Russian interference with a "10-foot pole" and no-one knew if Russia had tried to interfere with the 2016 EU referendum "because they did not want to know".
Mr Hosie told reporters: "There has been no assessment of Russian interference in the EU referendum and this goes back to nobody wanting to touch the issue with a 10-foot pole.
"This is in stark contrast to the US response to reported interference in the 2016 presidential election.
"There should have been an assessment of Russian interference in the EU referendum and there must now be one, and the public must be told the results of that assessment."
The report also said there had been "credible open source commentary" suggesting Russian tried to influence the Scottish independence referendum in 2014.
Committee member Mr Jones said the government had "let us down" by not looking into these accounts.
Analysis by Gordon Corera, BBC Security Correspondent
This report may not be what some expected, but it is still damning.
Many expected the committee to have answered the question of whether there was interference in political events like Brexit.
Instead, it says the problem was the government and the spy agencies failed to even look at this question.
British intelligence has, at least in recent years, been reluctant to get involved in anything that looks "political" and treated the issue of trying to protect democracy like a "hot potato".
But ultimately it's the government that the committee blames.
More broadly, there are serious questions about the failure of the UK to confront the spread of Russian money and influence over a long period.
And there is an urgent call for new legislation to deal with an ongoing challenge.