The former prime minister joined Ukrainian foreign minister Dmytro Kuleba at an online event calling for action against the Russian president.
He joined Ukrainian foreign minister Dmytro Kuleba at an online event organised by the Chatham House think-tank, where pleas were made for Western nations to set up a tribunal to prosecute the Russian president and his accomplices.
Mr Brown said the plan to set up a new international tribunal is modelled on the actions of the nations which met in London during the Second World War to draft a resolution on Nazi war crimes, which led to the creation of the International Military Tribunals and the Nuremberg trials.
Mr Brown said a new international tribunal is needed as well as existing international investigations by the International Criminal Court.
Mr Brown said: “Ukraine wants our full support to expose and punish the crime of aggression, and that can be done by setting up a special tribunal on the lines proposed in 1942.
“President Putin has posed a fateful challenge to the post-1945 international order. He has sought to replace the rule of law with a misuse of force.
“If we were to acquiesce in any way, none of us could ever take freedom or democracy for granted ever again.
“For all these reasons, and because of the scale of the suffering of the people of Ukraine, I believe that most people would agree that this act of aggression cannot go uninvestigated, unprosecuted or unpunished.”
The proposal Mr Brown is supporting has been formulated by senior international legal experts including Phillippe Sands QC, director of the Centre on International Courts and Tribunals at University College London; Philip Leach, professor of human rights law at Middlesex University; Dapo Akande, professor of public international law at the Blavatnik School of Government at Oxford University; and Murray Hunt, director of the Bingham Centre for the Rule of Law.
The proposal seeks to address a gap in the international legal infrastructure.
The lawyers are demanding that the UK and other countries join Ukraine to grant jurisdiction to a dedicated criminal tribunal to investigate both the perpetrators of the crime of aggression and those complicit in that crime.
Mr Brown added: “Currently the ICC can investigate crimes against humanity, genocide and war crimes. As evidence mounts acts against innocent civilians including children and the use of vapour bombs, it may be that Russia can be prosecuted for these crimes.
“But we lack a crucial extra weapon in the legal fight against Putin. because Russia has not signed up to a separate ICC statute under which nations pledge not to commit so-called ‘crimes of aggression’. We need the special tribunal.
“Mr Kuleba wants us to act and I believe we must do so now. Putin must not be able to escape justice.”