By Tuomas Forsell and Alastair Macdonald
HELSINKI/BRUSSELS (Reuters) - British Prime Minister 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 will struggle to secure major EU action at an EU summit next Thursday.
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, where the bloc's powerbrokers know European divisions run deep over how to handle President Vladimir Putin.
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 Sunday.
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 on Wednesday 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."
The relatively limited, national steps May announced do not put immediate pressure on allies to follow. Trade sanctions, requiring unanimous EU-wide backing, "won't fly", one EU official said, noting past opposition from the likes of Italy, Greece and Hungary to proposals for new sanctions.
A shift in Paris under Macron makes that even less likely, suggesting British diplomats will need to scope out other forms of measure which could form the basis of a summit agreement to demonstrate the solidarity which the EU wants to offer Britain.
Foreign Secretary Boris Johnson will take part in some of the groundwork for the summit when he is expected to brief fellow EU foreign ministers at a regular meeting on Monday.
Indicating one type of solidarity, NATO issued a joint call on Wednesday for Russia to explain the use of the Soviet-designed substance after a briefing from British diplomats.
Officials dismiss suggestions that May's intention to take Britain out of the EU in a year's time -- an issue also on the table at the summit -- will limit EU goodwill. Both sides have insisted mutual security must not suffer due to Brexit.
However, there is impatience with Britain's pursuit of its own interests and EU powers fear hampering efforts to work with Russia on problems such as Syria and Iran -- even while they also fret about Moscow's interference in EU politics.
One French official suggested Britain had itself to blame for a policy of taking in large numbers of wealthy Russians and Kremlin opponents and could not expect the EU to help: "Britain has tied itself up in knots," the official said. "It's shocking that it happened, but ultimately it's up to Britain."
(Additional reporting by Jan Strupczewski and Robin Emmott in Brussels and Luke Baker in Paris; Writing by Alastair Macdonald; Editing by Richard Balmforth)