Boris Johnson has attacked Russia for shutting down investigations into chemical weapons attacks in Syria after it blocked an independent probe.
Moscow, which backs Bashar Assad's brutal regime, used its veto at the United Nations to stop work to identify the perpetrators of atrocities being extended.
The move suggests the Syrian government "has something to hide" and risks letting the attackers "off the hook", the UK's Foreign Secretary said.
Russia accused the United States of calling the vote on renewing the mandate of the joint investigative mechanism (JIM) "to show up and dishonour Russia".
The JIM last year found the Syrian regime used chemical weapons against its people on at least three occasions.
It is due to publish a report shortly on two further attacks, including the Khan Sheikhoun atrocity in April that prompted a US retaliatory cruise missile strike on a Syrian air base.
The UK has said previously there is "no question" that Assad was responsible for the sarin gas attack.
Mr Johnson said the Government was "deeply disappointed" the JIM was not being renewed.
He added: "Russia's veto at the UN Security Council means that in a month the JIM will cease to exist. It will no longer be able to do its work to investigate who is responsible for use of chemical weapons in Syria.
"The veto suggests that the Syrian regime has something to hide and risks letting both Assad forces and Daesh off the hook for these heinous crimes.
"In 2013, Russia said it had secured Syria's agreement to destroy all its chemical weapons. Since then, the JIM has found the Syrian regime used chemical weapons three times. Russia's response is not to redouble its efforts to ensure disarmament, but to shut down the investigation.
"While we are deeply disappointed, the United Kingdom will not let Russia's actions stop us from working with international partners to seek justice for victims and to prevent the use of chemical weapons by anyone, anywhere."