The British investigation into the death of Aleksandr Litvinenko, a former Russian intelligence officer and Kremlin critic, who died from radioactive poisoning in 2006, is “biased” and unjustified, Russia’s UK ambassador Alexander Yakovenko told RT.

“Russia is not satisfied with the way the Litvinenko case was dealt with,” Yakovenko said in an interview to RT, stressing that “the key problem is… that it was not public and open to the press,” so that Russia could not take part in the hearings.

“That is why we cannot accept the conclusions of that case. If it was public, we would be happy to participate [in the investigation], but, unfortunately, it did not happen,” he added.

Yakovenko also emphasized that the results of the inquiry “are not justified,” with the investigation itself being “very politicized” and “biased” as “the coroner investigation was open to [Russia] but later, after [the crisis in Ukraine started], the British authorities decided to switch the process to a secret investigation…”

“In order to prove something, you have to present the facts. As soon as the British side proves… their conclusions, we will be ready to consider [them],” the ambassador stressed adding that now he sees “no grounds for that,” as the Russian side “did not even have a chance to study the documents [of the investigation].”

Yakovenko also said that “the allegations that the Russian State, in one way or another, is involved in the death of Litvinenko” were “absolutely unacceptable.”

According to the ambassador, the way the investigation was done “undermines bilateral relations as well as trust” between the two countries and could be “a major problem for the Russian-British relations.”

At the same time, he stressed that it is just one of “a number of differences” in relations between the two nations as the UK and Russia already have different views on the Ukrainian crisis as well as on the Syrian peace process with the UK refusing to cooperate with Russia in Syria and to share intelligence on terrorists in the country preferring to “blame Russia for bombing [the] wrong targets.”

Earlier on Thursday, the UK presented the results of a public inquiry into the death of Aleksandr Litvinenko, accusing senior Russian officials, including Vladimir Putin, of “probably” approving the murder.

The chairman of the inquiry, Sir Robert Owen, said President Vladimir Putin “probably” approved the operation to assassinate Litvinenko – an outspoken critic of the Kremlin and associate of exiled Russian billionaire Boris Berezovsky – based on a personal grudge.

The public inquiry into the case was launched in January 2015, almost a decade after Litvinenko died from radioactive poisoning in 2006. The case cannot face a formal trial in Britain as the main suspects are not in the UK.

Russian officials, as well as the two men suspected in Britain of killing Litvinenko – Andrey Lugovoy and Dmitry Kovtun – have always denied the accusations.

Later on Thursday, Home Secretary Theresa May said that the UK would summon the Russian ambassador to ‘express displeasure’ over a lack of cooperation in the investigation and called the murder “a breach of the international law.”

Prime Minister David Cameron’s office said the British government is considering further action against Russia.