The widow of a Russian dissident murdered in London has backed a legal challenge to force Prime Minister Boris Johnson's government to publish a report on alleged Russian meddling in British politics.

The report by parliament's Intelligence and Security Committee (ISC) was cleared by Britain's security services but Johnson's office has not yet released it, meaning it will not be published before the Dec. 12 election as parliament has shut to allow for campaigning.

Marina Litvinenko, whose husband Alexander, a former Russian spy, was murdered with a radioactive isotope in London in 2006, sent a letter to Johnson last week warning that she would take legal action to force publication.

She said she had now thrown her support behind another court challenge led by the Bureau of Investigative Journalism, her lawyers said in a statement.

"Mrs. Litvinenko is extremely disappointed with the prime minister’s hollow reasoning and politically opportunistic decision to maintain his refusal to publish the ISC report, notwithstanding the cross-party and public demand for this action," the statement said.