Reports say Moscow is putting "potentially the most significant threat to UK's institutions and its ways of life," and UK governments have turned a blind eye to it as it might contain embarrassing revelations about Prime Minister Boris Johnson and his party.

According to CNN, Moscow built up a network of British diplomats, officials and other influencers across the political spectrum.

Members of the cross-party Intelligence and Security Committee (ISC), watchdog of British intelligence and security agencies, made an unpublished final report into Russian meddling in UK politics entitled simply "Russia," is at the center of a storm in the UK, as CNN reported. The chief of the committee Dominic Grieve has accused Johnson of ignoring the report and claimed Downing Street had given "bogus" explanations for not publishing it.

The report by parliament's Intelligence and Security Committee (ISC) has been cleared by the security services but it has not yet been given approval for publication by Johnson's Downing Street office, meaning it cannot be released before a December 12 election. Sources have told CNN the report includes a heavily redacted annex.

Britain has accused Russia of meddling or trying to interfere in Western elections, accusations denied by Moscow. The ISC was examining allegations of Russian activity aimed at the United Kingdom, including in the 2016 referendum on EU membership, when Johnson was a leading campaigner to leave.

Emily Thornberry, foreign affairs spokeswoman for the opposition Labour Party, said the decision not to publish the parliamentary committee's report was for "utterly unjustifiable, unprecedented and clearly politically-motivated reasons."

"What is Downing Street so worried about? I fear it is because they realize this report will lead to other questions about the links between Russia and Brexit," she told parliament.

She said questions might be raised about Johnson's relationship to a suspected Russian spy who described the prime minister as a "good friend."

The report might also raise questions about connections of Johnson's senior aide Dominic Cummings, who worked in Russia in the 1990s, and about money from Russian sources to Johnson's Conservative Party, she said.

(With input from Reuters)