The new Labour government wants to inject renewed energy into the two-year-long international effort to set up a special tribunal with the authority to try Russia’s leadership for the crime of aggression, the lord chancellor, Shabana Mahmood, has said. Discussions have been dogged by disputes over the appropriate body to set up the special tribunal, and fears in the US that if an organisation were empowered to strip the Russian leadership of immunity from prosecution in a foreign court, western leaders might face the threat of legal action in the future. The UK has been one of the driving forces in the core group of 40 countries looking to find a mechanism to hold the three Russian leaders – Vladimir Putin, prime minister Mikhail Mishustin and foreign minister Sergey Lavrov – to account. Mahmood told the Guardian that urgent action needed to be taken – not just on the military front but also in the legal sphere – over what has happened to Ukraine. “We want to inject some energy into the process because it has been going on too long,” she said. “We want to work this through very carefully, but at pace, to unlock what is the best mechanism to deliver the special tribunal.” She was speaking on the eve of a meeting of justice ministers from the Council of Europe in Vilnius that will discuss how to progress the discussions on the format for a Nuremberg-style international tribunal that can secure global legitimacy. It is the first time that Labour in government has addressed the issue of putting Putin on trial for the invasion of Ukraine. Mahmood said she was aware there was strong support in Ukraine and elsewhere for the idea that the Council of Europe should take a central role in establishing a special tribunal. “As a new administration,” she said “all options are on the table”. She added that there was no dampening down of Ukrainian enthusiasm for this system of accountability to be set up. Alicia Kearns, the shadow Foreign Office minister, said this week that during David Cameron’s tenure as Conservative foreign secretary, the international consensus had been leaning towards “a special tribunal, held in a third country, established by a treaty between the Council of Europe and Ukraine, and supported by a wider membership through a vote in the UN general assembly”. She called for a vote on the issue at the general assembly later this month, Mahmood said the government wanted to find a mechanism that “delivers, meets the legal challenges and is effective” without slowing the process. She added: “We are very committed to the Council of Europe and the European convention on human rights, and that has never been more important than now because of what is happening in Ukraine.” The case for a special tribunal has been pushed hardest in the UK by the human rights barrister Philippe Sands KC and the former prime minister Gordon Brown. It is argued that the route to a special tribunal through the United Nations security council is blocked due to the existence of the Russian veto, making the Council of Europe the best alternative. The international criminal court (ICC) is not in a position to try the crime of aggression, as opposed to individual war crimes, since neither Russia nor Ukraine have yet fully ratified the Rome statute that founded the ICC. Although the ICC has issued arrest warrants for Putin for his role in the abduction of Ukrainian children, and for the former defence minister Sergei Shoigu over the bombing of Ukraine’s electricity infrastructure from October 2022 to March 2023, it is argued that it cannot try the three key members of the Russian leadership that directed the invasion of Ukraine in February 2022. The ICC prosecutor, Karim Khan, held talks with Mahmood this week. He has in the past questioned the value of a special tribunal if it duplicated the work of the ICC.