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Boris Johnson has been accused of giving the “green light” to Russian interference in UK politics by refusing to assess whether Russia tried to influence the 2016 Brexit referendum.

The prime minister has rejected one of the key recommendations in the long-awaited Russia report – to publish an assessment of whether Moscow tried to influence the EU vote.

The intelligence and security committee made the recommendation after finding that Tory ministers and intelligence agencies did not do enough to investigate or protect the UK from possible Russian interference in the referendum.

But the government said that with no evidence of interference to date, a “retrospective” assessment was “not necessary”.

The government’s refusal was sharply criticised, with Johnson facing allegations that he has exposed himself “to the charge of complicity in distortion of our democratic process”.

Acting Liberal Democrat leader Ed Davey said: “Boris Johnson refusing to launch an inquiry into Russian election interference is a green light for them to do it in the future. 

“The government has been told by a cross-party group of MPs that we must establish what role Russian interference played in the Brexit referendum in order to protect our democratic process, but still refuse to pursue it.

“You cannot hope to deal with interference if you do not even check if it is happening. 

“Rather than sticking his head in the sand, Boris Johnson must ask the intelligence community to launch a wide-ranging and properly funded investigation so that we can understand just what influence Russian interference had on our democracy.” 

Tory former cabinet minister Stephen Dorrell, who now chairs the European Movement, said: 

“The report’s conclusion that the government ‘did not take action to protect the UK’s process in 2016’ is extraordinary. 

“The European Movement does not challenge the result of the 2016 referendum, but it does challenge the government to expose the extent to which Russia sought to influence the outcome. 

“Failure to do so exposes the government to the charge of complicity in distortion of our democratic process.

“Those most in favour of the Brexit project love to speak about British sovereignty – yet fall remarkably silent when presented with concrete evidence of foreign interference in our democracy. 

“The foreign secretary speaks of ‘hostile actors’; this report challenges him to follow up his words with actions.”

SNP defence spokesperson Stewart McDonald said: “During the election campaign, Boris Johnson claimed ‘there is absolutely no evidence that I know of to show any interference in any British electoral event’. 

“The report – which the prime minister had already read at that point – now makes clear that this is because the Tory government was not looking for it. 

“This botched attempt at a cover up by Downing Street has been laid bare by the committee’s findings.”