Police officers seal off a cul-de-sac in Salisbury, England, near to the home of former Russian ex-spy Sergei Skripal as a nerve agent is believed to have been used to critically injure him and his daughter Yulia. Picture: Andrew Matthews/PA via AP

Paris - France said on Thursday it agreed with Britain that Russia was behind a nerve agent attack on a Russian ex-spy in England, President Emmanuel Macron's office said in a statement.

"Since the beginning of the week, the United Kingdom has kept France closely informed of the evidence gathered by British investigators and evidence of Russia's responsibility in the attack," the president's Elysee office said, after a telephone call between Macron and British Prime Minister Theresa May.

"France agrees with the United Kingdom that there is no other plausible explanation and reiterates its solidarity with its ally."

Former spy Sergei Skripal, 66, and his daughter Yulia, 33, remain in hospital in critical condition after being found unconscious on a bench in the city of Salisbury on March 4. A police officer was also harmed and remains in a serious condition.

British Prime Minister Theresa May said on Wednesday that "there is no alternative conclusion, other than that the Russian state was culpable for the attempted murder of Mr. Skripal and his daughter, and for threatening the lives of other British citizens."

Russia has denied any involvement in the nerve agent attack.