Theresa May has renewed her war of words with Moscow over the nerve agent attack in Salisbury.
The Prime Minister again accused Russia of being behind the “sickening” incident.
Mrs May made the remarks as she thanked Nordic leaders for their support for the UK’s position in the aftermath of the attack.
Britain blamed the Russian GRU military intelligence agency for the poisoning of ex-spy Sergei Skripal and his daughter Yulia with Novichok in the Wiltshire town in March.
Addressing a meeting of the Nordic Council in Oslo, the PM said: “We believe that global stability, security and prosperity depend on adherence to the well-established international system of rules, treaties and protocols.
“And we recognise that it is not enough to simply hold such beliefs.
“We must be prepared to speak and act in their defence.
“We saw that earlier this year when the Russian state deployed chemical weapons in my country.
“And all five Nordic governments stepped up to stand alongside the UK.
“The sickening attack was systematic of a period in which the rules and norms that underpin international behaviour are being tested and challenged by both malign states and non-state actors.
“Permitting such a degradation of the rules-based order risks an increase in global instability.
“It makes it all the more difficult for nations to co-operate with one another to tackle common challenges.
“We must work together to take practical steps in defence of the international order and in defiance of those who would undermine it.
“That can be through diplomacy as the UK and Nordic world showed after Salisbury.”
Russia has also been blamed for the death of 44-year-old Dawn Sturgess and the poisoning of her partner Charlie Rowley, who are believed to have come into contact with Novichok discarded by the Skripals’ attackers.