Clinton said there was "no doubt" that Russia is "determined to try to shape the politics of western democracies".
"Not to our benefit, but to theirs," said the Democrat, who lost the 2016 US election to President Donald Trump in a race that US intelligence chiefs have said was marred by Russian interference.
"I find it inexplicable that your government will not release a government report about Russian influence. Inexplicable and shameful," she told BBC Radio 5 Live. Britain's parliament was previously told a report by the Intelligence and Security Committee was sent to the prime minister for approval on October 17.
The probe into suspected Russian covert actions in Britain's democratic process reportedly includes examining whether Moscow tried to interfere with the 2016 Brexit vote and the 2017 general election.
Moscow has been accused of spearheading sophisticated disinformation campaigns around the world to further its interests.
The Intelligence and Security Committee, which oversees the work of the country's intelligence agencies, submits its reports to the government before publication to avoid the inadvertent release of sensitive information.
A former head of domestic spy agency MI5 and the committee's chairman have suggested Johnson is stalling on the release of the 50-page report with the December 12 general election looming.
Jonathan Evans, head of MI5 from 2007 to 2013, called on the government to explain the delay.
Foreign Office minister Chris Pincher has previously said that the report is going through "an intensive security review".
Pincher has also played down concerns, saying there is "no evidence" to suggest there has been any successful Russian involvement in the British electoral system.