LONDON (Reuters) - 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.

FILE PHOTO: Marina Litvinenko, widow of former Russian intelligence agent Alexander Litvinenko, poses for a portrait during an interview with Reuters in London, Britain, March 14, 2018. REUTERS/Toby Melville/File Photo

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(ISC) was cleared by Britain’s security services but Johnson’s office has not yet released it, meaning it will not be published before the Dec. 12 election as parliament has shut to allow for campaigning.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

Britain has accused Russia of meddling and trying to interfere in elections in the West, which Moscow denies.

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 Leave campaigner.

The Sunday Times newspaper reported this month that the report would name nine Russian business people who gave money to Johnson’s Conservative Party.

Johnson and the government have said there was nothing unusual about the timing of the report’s publication and it was not being suppressed.

According to a British public inquiry in 2016, two Russians carried out Litvinenko’s murder, one of them a former KGB bodyguard who became a Russian lawmaker, in an operation probably ordered by Russian President Vladimir Putin.

“If the ISC report shows there is Russian interference in British politics, and that this led to a weak response to (her) husband’s murder, the public should know about it as soon as possible,” the statement from Marina Litvinenko’s lawyers said.