The United States, Germany and France formally backed Britain's claims of likely Russian links to a chemical toxin attack against a former spy, calling it the "first offensive use of a nerve agent" in Europe since World War II.

In a rare joint statement, the leaders of the four countries said the attack threatened "the security of us all". They called on Russia to provide "full and complete disclosure" of the Soviet-era chemical programme that developed the nerve agent Novichok.

Russia has denied any involvement in the poisoning, and vowed a swift response to Britain's expulsion of its diplomats.

Russian Deputy Foreign Minister Sergei Ryabkov said yesterday that neither Russia nor the Soviet Union had run a programme to develop the Novichok nerve gas.

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