The UK ordered 23 Russian diplomats back to Moscow last week after the country concluded that Russia was responsible for the poisonings.
Macron told a joint news conference with Merkel after the summit in Brussels that the March 4 incident - for which Russian Federation denies responsibility - was "a serious challenge to our security and... an attack on European sovereignty".
"The exact number is now being discussed with our European partners and will be announced on Monday".
France's Foreign Minister Jean-Yves Le Drian, on a visit to Kiev, signalled that Paris was considering expelling Russian diplomats in solidarity with Britain.
The EU said its ambassador to Russian Federation was recalled "for consultations".
This newfound unity among European Union leaders on spy poisoning matter and the appointment of John Bolton as President Donald Trump's national security advisor, who is considered as fierce Russian Federation hawk, are potential precursors of collective Western action against Russian Federation.More news: 'Legends Of Tomorrow' Adds Matt Ryan As Series Regular For Season 4
Meanwhile, a convoy of vehicles left the British Embassy in Moscow on March 23, ahead of Moscow's deadline for 23 British diplomats to leave the country, Reuters television footage showed.
Diplomatic pressure is growing on Moscow over its role in the nerve agent attack on Sergei Skripal and his daughter Yulia in the English city of Salisbury in early March. They remain in a critical but stable condition after the attack on March 4.
British Prime Minister Theresa May arriving for the second day of the European Council's Spring meeting in Brussels, Belgium.
French President Emmanuel Macron said Friday the poisoning with the Soviet-made "Novichok" agent was an "attack on European sovereignty", after EU leaders unanimously backed London's assessment that it was "highly likely" Moscow was to blame and that there was no plausible alternative explanation.
The president of Lithuania, Dalia Grybauskaitė, said that "from the beginning of next week, a lot of countries, we will go for our national measures".
In response, Russia's foreign ministry said: "We regret that the EU chose to follow another anti-Russian campaign launched by London and its allies from across the ocean, with the obvious objective of creating yet another obstacle to normalising the situation on the European continent".More news: SNOW: Somerset schools re-open as weather improves
Czech Prime Minister Andrej Babis said he was likely to announce the expulsion of several people on Monday, after returning to Prague and consulting with his foreign minister.
She said in the run-up to the summit Britain had been sharing what information it could through "intelligence channels" as it sought to make the case for Russian responsibility.
The issue of relations with Russian Federation divides European Union nations sharply, and there is pressure from some to lift economic sanctions against Moscow, so Britain has not pressed for additional steps of that kind.
"We stand in unqualified solidarity with the United Kingdom in the face of this grave challenge to our shared security", the statement by the European Council said.
Meanwhile, British diplomats expelled from Russian Federation have left the embassy in Moscow ahead of Russia's deadline.
The leaders of France and Germany declined to say whether they also planned expulsions, but said more action was needed against Russian Federation.More news: India says 39 workers abducted in Iraq in 2015 dead
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