Not for the first time in recent history, Russia appears to have committed murder or attempted murder on British soil. The poisoning of a former KGB agent Alexander Litvinenko in London in 2006 shocked the world but nothing was done about it. Now another former Russian agent, Sergei Skripal, has been found unconscious and near death on a public seat in Salisbury with his daughter.

The cause has been identified as a secret military-grade nerve agent known to be developed by Russia and kept in its possession. Prime Minister Theresa May has declared it "highly likely" the Kremlin is responsible and this time Britain sounds determined to do something about it.

May has elicited the support of Nato allies Germany, France and the United States. A White House spokeswoman said Donald Trump agrees the Russian Government must explain how its nerve agent came to be used. May set an overnight deadline which Russia said it could not meet unless Britain provided a sample of the stuff.

Russia looks certain to face another set of sanctions over this, and so it should. It seems to think it can enter other countries and exact vengeance on turncoats at will. Skripal was a double agent, caught in Russia and imprisoned there until released to Britain in a prisoner exchange more than seven years ago.

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It is not known why an attempt has been made to kill him now but as May said, "This attempted murder using a weapons-grade nerve agent on a British town was not just a crime against the Skripals, it was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk."

Almost 40 people may have been at risk if the chemical was delivered by spray, but the principle Russia has breached is just as important. For agents of a state to cross the borders of another state and carry out an attack on anyone, is an outrage to civilised international conduct.

It is an outrage this country experienced over the Rainbow Warrior which was not an intentional killing. This most certainly was intentional and might yet succeed. If the Skripals do survive they are likely to be crippled.

It is in the interests of all nations, and of international order, that national borders are respected even by spy agencies. And by large, borders are respected in espionage. Double agents were able to find safe haven in their adopted country during the Cold War but Putin's Russia appears to observe no such conventions.

There is not much civilised states can do in response but trade, sporting and diplomatic sanctions do count. As a writer in the Washington Post noted two years ago, when a British inquiry found Litvinenko's murder was probably approved by Putin, "The story of the past few years has been his steady descent from a leader welcomed and even feted around the world, to one whose hand is shaken only reluctantly."

Theresa May was Home Secretary when that finding was made. She told Parliament then Russia would face a tough response. This time she is in a position to see that it does. All countries should support her.