UK Prime Minister Theresa May visits Salisbury city centre on March 15, 2018, where Russian Sergei Skripal was attacked with a deadly substance and remains in hospital alongside his daughter Yulia. [Photo: IC]

The spy poisoning row escalated on Thursday as the leaders of France, Germany, the United Kingdom and the United States issued a joint statement to blame Russia for a chemical attack on a former Russian spy, a charge denied by Moscow.

"We, the leaders of France, Germany, the United States and the United Kingdom, abhor the attack that took place against Sergei and Yulia Skripal in Salisbury, UK, on March 4, 2018," the statement said.

The four nations called upon Moscow to provide full and complete disclosure of its Novichok nerve agent program to the Organization for the Prohibition of Chemical Weapons.

Novichok is the military-grade nerve agent which was found on the scene of the poisoning incident, according to the British government.

Russian Foreign Ministry spokeswoman Maria Zakharova said Thursday that Britain had refused to cooperate with Russia "on the fulfillment of its duties as a signatory to the Chemical Weapons Convention."

Moscow will soon take retaliatory measures against Britain regarding the spy attack row, she said.

British Prime Minister Theresa May on Wednesday announced a series of measures against Russia over its failure to respond to demands by the British government to explain how a military-grade nerve agent was used in the spy attack.

These measures included the expulsion of 23 Russian diplomats in a week, the freezing of Russian state assets in Britain, the suspension of all planned high level bilateral contacts and the boycott by ministers and the royal family to attend the FIFA World Cup in Russia.

The London-Moscow row started following the murder attempt on former Russian double agent Sergei Skripal and his daughter Yulia in Britain's southwestern city of Salisbury.

Found unconscious on a bench outside a shopping center there on March 4, they remain in critical condition.

Britain claims the father-daughter pair was exposed to a nerve agent and Russia is responsible for the act, which Moscow denies.