Labour has condemned Vladimir Putin's decision to withdraw Russia from the International Criminal Court, which rules on charges such as genocide and crimes against humanity.
But Downing Street said that the decision was a matter for Russia, and that Prime Minister Theresa May's focus would be on continuing to urge Moscow to uphold humanitarian law in conflict zones like Syria.
Mrs May's spokeswoman pointed out that, while the Russian president signed the treaty establishing the Hague-based court, Moscow has never ratified it and was never a full member.
Mr Putin's decree, published on the Kremlin's website, came a day after the UN General Assembly's human rights committee approved a resolution condemning Russia's occupation of Crimea as well as abuses including discrimination against some of the Ukrainian peninsula's population.
For Labour, shadow foreign secretary Emily Thornberry said: "We utterly condemn this appalling decision, and its deplorable and cynical timing.
"It is a severe blow to all those who want to see international justice upheld around the world, and the fact that not all countries are currently bound by the ICC is no excuse for those that are signatories to abandon it.
"We call on the Russian government to reconsider, and to work with the international community to defend the ideal of universal justice."
The announcement was "particularly alarming" because of this week's resumption of air strikes on the Syrian city of Aleppo, she said.
Asked about Mr Putin's move, Mrs May's spokeswoman said: "It is a decision for Russia. It's important to be clear that Russia had never ratified the Rome Statute so they were already not a full member of the ICC, so in that sense they are not withdrawing from it.
"From the UK's perspective, we will continue to stand up for a rules-based international system, in particular upholding international law in scenarios, events and situations on the ground where we see it - for example with the work we have been doing to urge Russia to respect international law in Syria."
Britain continues to regard the annexation of Crimea as illegal, said the spokeswoman.
And she added: "We think the ICC plays an important role upholding the rule of law and holding people to account for serious international crimes."