"The UK will call a UN Security Council meeting tomorrow on Ukraine," Barbara Woodward, the UK's Permanent Representative to the United Nations, said in a tweet on Monday.
"We will discuss the mounting evidence of war crimes, including from Bucha," she added.
"We'll use our Presidency of the Security Council to ensure there is transparency, accountability and that justice is done," Woodward also said.
Meanwhile, Britain's Foreign Secretary Liz Truss tweeted: "Given strong evidence of war crimes, including reports of mass graves and heinous butchery in Bucha, Russia cannot remain a member of the UN Human Rights Council."
UK Prime Minister Boris Johnson said that the "discovery of more mass graves outside Kyiv is sickening."
He pledged that Britain "will not stand by whilst this indiscriminate and unforgivable slaughter takes place."
Johnson added: "We are working to ensure those responsible are held to account. We will not rest until justice is done."
Polish Prime Minister Mateusz Morawiecki on Monday called for the establishment of an international committee to investigate what he called the crime of genocide committed by Russian forces in Ukraine.
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Source: PAP, Reuters