And she will call on the EU and Nato to "stand shoulder to shoulder" in defiance of Vladimir Putin's "brazen actions".

"The challenge of Russia is one that will endure for years to come," the Prime Minister Theresa May is expected to tell the gathering of the EU's 28 national leaders.

"As a European democracy, the UK will stand shoulder to shoulder with the European Union and Nato to face these threats together," she will add.

She will also promise: "United we will succeed."

Mrs May will make her plea for a European united front against the threat of Russia during a discussion of the Salisbury incident at the EU Council meeting in the Belgian capital.

Whitehall officials are understood to be determined to emphasise the need for "shared security" among Western nations following the first deployment of a chemical weapon on European soil since the second world war.

"Russia has shown itself to be a strategic enemy, not a strategic partner," said one senior Whitehall source.

Diplomats are said to have passed highly classified details of the make-up of the Novichock nerve agent to Britain's European allies to show the evidence of suspected Russian state culpability in the attack.

British officials have also pointed out to European counterparts that Russian government spokesmen have made 17 different attempts to explain away the allegations including suggesting the victims were attempting suicide or blaming other countries including Ukraine and Sweden.

At the summit tonight, Mrs May will describe the attack as "attempted murder using an illegal chemical weapon known to be possessed by the Russian regime", according to a British Government official.

She will say the poisoning was an “indiscriminate and reckless attack” against the UK which put the lives of innocent people at risk.

Leading members of the international community shared the UK's assessment that Russia was responsible and had joined the call for a full explanation of the "brazen actions", the Prime Minister is expected to say.

She will point out that the use of a nerve agent was a "clear violation" of the chemical weapons convention and a breach of British and international laws as well as British values.

Disrespect for international rules and norms clearly threatens the basis for our advanced democracies, open societies and free economies, she will argue.

Mrs May will warn her European counterparts that the Russian threat does not respect borders, saying: "We are all at risk."

She will set out how Russian authorities have responded to British allegations with disdain and disinformation and had failed to meet their country's chemical weapons obligations.

The Government official said: "We want to work with our EU allies to uphold and protect the international rules-based order to hold Russia to account for this flagrant breach of national laws, to ensure such a heinous crime is never repeated and to protect our shared security in the face of the long-term challenges that Russia poses."