A British inquiry into the 2006 death of Alexander Litvinenko has found that the killing of the former Russian spy was “probably approved” by Russian President Vladimir Putin and the head of Russia’s intelligence agency.

The results of public inquiry, published Thursday, suggest that Putin and Nikolai Patrushev, then the head of FSB, the Russian intelligence agency that succeeded the KGB, signed off on the operation, wrote Judge Robert Owen.

Taking full account of all the evidence and analysis available to me, I find that the FSB operation to kill Mr. Litvinenko was probably approved by Mr. Patrushev and also by President Putin,” Owen said.

Litvinenko signed a statement two days before his death that alleged the Russian president had ordered his murder. Thursday’s report is the first public official statement linking Putin to the crime.

Litvinenko, a former KGB agent turned Kremlin critic who fled to Britain, fell suddenly ill in November 2006, a month after drinking tea with two Russian agents in a London hotel. Hospital tests detected the presence of a radioactive poison, polonium-210, in his body. Litvinenko, 44, died three weeks later.

Moscow has always denied involvement, and its reaction to the new report was no different. “We regret that a purely criminal case has been politicized and has darkened the general atmosphere of our bilateral relations,” said Russian foreign ministry spokeswoman Maria Zakharova, according to The Guardian.

Russia has for years denied extradition requests by Britain for Andrei Lugovoy—who is now a member of the Russian parliament—and Dmitry Kovtun, the two men Litvinenko met in the bar of the Millennium Hotel in 2006 and whom British prosecutors believe poisoned Litvinenko’s tea. In the report, Owen wrote that, given the results of the inquiry, he is “sure” that Lugovoy and Kovtun placed polonium-210 in Litvinenko’s drink.

British Home Secretary Theresa May on Thursday called the crime a “blatant and unacceptable” breach of international law, and said British Prime Minister David Cameron would speak to Putin about the report at “the next available opportunity,” according to the BBC.

Marina Litvinenko, the former agent’s widow, praised the “damning” findings Thursday, and called on the prime minister to impose sanctions on Putin and other officials and expel Russian intelligence agents working in Britain, the Associated Press reported. Her lawyer called her husband’s death “a mini-act of nuclear terrorism on the streets of London.”

Marina Litvinenko has said her husband grew disillusioned with the FSB in the 1990s, during Russia’s war against Chechan separatists. He fled Russia to Britain in 2000 and was granted asylum, and became a vocal critic of Putin, claiming at one point that the Russian president was a pedophile. Litvinenko wrote two books about what he saw as corruption within Russia’s intelligence agency, and accused Russia of carrying out the bombings of apartments in several Russian cities in 1999, which killed more than 300 people. Litvinenko eventually worked for MI6, Britain’s main foreign intelligence service.

“There was undoubtedly a personal dimension to the antagonism between Mr. Litvinenko on the one hand and President Putin on the other,” Owen wrote. “The history between the two men dated back to their (only) meeting in 1998, at a time when Mr. Putin was the newly appointed head of the FSB.”

The 328-page report was the product of testimony of 62 witnesses from six months of public hearings and Owen’s private viewing of secret intelligence evidence about Litvinenko.

Litvinenko’s death drove a wedge between Russia and the United Kingdom for years. The public inquiry was launched in January of last year, months after Britain imposed economic sanctions against Russian individuals for their involvement in the annexation of Crimea. The findings will likely create a diplomatic headache for both world powers in areas that depend on cooperation;  the two nations are now fighting the same terrorist threat in the Middle East, and British diplomats believe a resolution to Syria’s five-year-long civil war can’t be reached without Russia, an ally of the country’s president, Bashar al-Assad.