Theresa May has a week to rally wary European Union leaders behind punitive measures against Moscow over the poisoning of a Russian former spy.

But the British Prime Minister will struggle to secure major EU action.

Despite talk of "full solidarity" with London after a "brutal attack inspired most likely by Moscow" from summit chair Donald Tusk and others in Brussels, there is profound caution in Paris and Berlin.

The bloc's powerbrokers know European divisions run deep over how to handle Russian President Vladimir Putin.

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The French Government questioned May's evidence for pinning the Salisbury attack with Soviet nerve agent on Moscow and German Chancellor Angela Merkel warned of a need to keep talking to a Russian president set to be re-elected easily on Monday.

EU diplomats virtually rule out any deal to ramp up economic sanctions which would take a unanimous vote.

As a result they do not expect May to ask for that, though she said today she would seek allies help and might take further measures after expelling Russian diplomats and announcing other limited action.

Tusk confirmed that he was ready to put the issue on the agenda for the regular March 22-23 summit in Brussels, although EU officials said May had yet to request that.

The former Polish premier declined to say what measures leaders may consider after four years of bickering about whether sanctions over Russian action in Ukraine are counter-productive and hurt EU businesses.

France, where Macron has taken a softly-softly line with Putin since taking office last year, said it wanted to see definitive proof that Moscow was behind the attack.

Merkel said she took May's accusations of Moscow's involvement "very seriously", but added that dialogue had to be maintained with Russia "despite all differences of opinion".

As the EU works on a common stance ahead of the summit, she said: "We can't break off all contacts now. We must still talk with the Russians despite all differences of opinion."

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- Reuters, AAP