Macron backs UK claim that Russia behind spy attack

French President Emmanuel Macron today backed Britain's charge that Russia was responsible for the nerve agent poisoning of a former spy, saying there was "no other plausible explanation".

"Since the start of the week, Britain has kept France closely informed of the evidence gathered by British investigators and of elements demonstrating Russian responsibility in the attack.

"France shares Britain's assessment that there is no other plausible explanation and reiterates its solidarity with his ally," Macron's office said after today's morning call between the French leader and British Prime Minister Theresa May.

Macron and May condemned "all use of chemical weapons" and agreed "on the importance of European and transatlantic unity" in response to the attack, the French presidency added in a statement.

The statement did not say what kind of measures Britain's allies might adopt but the two leaders agreed to "remain in close contact in the coming days".

A British diplomat, speaking on condition of anonymity, said London was "grateful for French solidarity at this time".

"As President Macron said this morning, the unity of our European and transatlantic alliance is essential as we respond to Russian behaviour."