London: Questions about the British government's failure to release a report on Russia's interference in the country's politics have continued to dog Prime Minister Boris Johnson as critics said leaks from the document raised concerns about the security of next month's election.

British Prime Minister Boris Johnson speaks to staff and students at Bolton University in England on Saturday. AP

The report from Parliament's intelligence committee concludes that Russian interference may have affected the 2016 referendum on Britain's departure from the European Union, though the impact is "unquantifiable," the Sunday Times reported.

The committee said British intelligence services failed to devote enough resources to counter the threat and highlighted the impact of articles posted by Russian new sites that were widely disseminated on social media, the newspaper reported.

Emily Thornberry, the opposition Labour Party's foreign affairs spokeswoman, said the leaks raise questions that deserve answers ahead of the December 12 poll.

"Boris Johnson therefore needs to clear up the confusion, spin and speculation around this [intelligence committee] report by publishing it in full at the earliest opportunity," she told the Times. "If not, people will rightly continue to ask: what is he trying to hide from the British public and why?"

Johnson's government has said it needs more time to review the security implications of the report. It says it will release the report after the election.

British Prime Minister Boris Johnson launches election campaign with a speech outside No 10.

Critics have alleged the report is being withheld because it shows Russians have made large donations to the Conservative Party, which is seeking to win a majority that would allow Johnson to push his Brexit deal through Parliament.

Security Minister Brandon Lewis dismissed the criticism.

Asked about Russian donors to the campaign, Lewis told Sky television on Sunday that all contributions are reported to the proper authorities and the donors in question all have British citizenship.

"We should not prevent British citizens from taking and playing their full part in British political life," he said.

Former Special Counsel Robert Mueller's investigation into the 2016 US presidential election found that Russia interfered in the vote in a "sweeping and systemic" fashion.

While President Donald Trump dismissed the findings, the US investigation put Russia at the centre of worries about the integrity of elections worldwide.

The House of Commons Intelligence and Security Committee began its investigation following allegations of Russian interference in both the 2016 US election and the Brexit referendum earlier that year.

The committee sent its report to Johnson for review on October 17, saying it expected to "publish the report imminently."

Committee Chairman Dominic Grieve has criticised Johnson's government for failing to release the document amid media reports it has already been cleared by British security services.

The debate comes amid growing concerns about the security of elections fought in an increasingly digital world. Britain's election laws were written for a time when campaigns pushed mass-produced leaflets through mail slots, rather than flooding Facebook and Twitter accounts with individually targeted messages.

Johnson called the early election in response to the political turmoil caused by Britain's pending departure from the EU.

AP