Deputy Prime Minister and Justice Secretary Dominic Raab was speaking as details of a war crimes summit, involving justice ministers from across the world, was confirmed.
Karim Khan, International Criminal Court prosecutor, is investigating accusations that Russian soldiers have carried out war crimes during Moscow's invasion of Ukraine.
They include allegations from Kyiv that civilians have been raped and murdered during the 11-month conflict.
“Russian forces should know they cannot act with impunity and we will back Ukraine until justice is served,” Mr Raab said.
“Almost a year on from the illegal invasion, the international community must give its strongest backing to the ICC so war criminals can be held to account for the atrocities we're witnessing.”
Justice ministers will meet in London in the spring; Ukraine and the war crimes investigation will top of the agenda.
“The reports and images of Russia's unlawful and unprovoked armed attack on Ukraine are horrific,” said Dilan Yesilgoz-Zegerius, Dutch Justice Minister and co-host of the war crimes meeting.
“For us it is crystal clear — these crimes may not go unpunished.”
The group will hear from Mr Khan about how the international community can support its investigations.
Officials said this could include offering practical support, such as helping to gather information and share evidence of atrocities committed in Ukraine.
Ministers will also discuss how to help victims and witnesses provide testimonies, without causing them further distress.
Last year the UK offered a package of support to the ICC, which included an additional £1 million ($1.2 million) in funding plus dedicated police assistance.
The Ministry of Justice said it had also been involved in training Ukrainian judges set to conduct war crime trials and has offered the support of British legal experts.