Home Secretary Sajid Javid has called on Russia to explain the Novichok poisoning after two people were exposed to it in Wiltshire.
Britain called on Russia to give details about the Novichok nerve agent attack on a former double agent and his daughter after two British citizens were struck down with the same poison.
The two Britons, a 44-year-old woman and a 45-year-old man, were critically ill after an apparently chance encounter with the poison near the site of the March attack on ex-double agent Sergei Skripal and his daughter Yulia.
Britain accused Russia of poisoning the Skripals with Novichok - a nerve agent developed by the Soviet military during the Cold War - in what is the first known offensive use of such a chemical weapon on European soil since World War Two.
Russia, which is currently hosting the soccer World Cup, has denied any involvement in the March incident and suggested the British security services had carried out the attack to stoke anti-Moscow hysteria.
“The Russian state could put this ‘wrong’ right. They could tell us what happened, what they did and fill in some of the significant gaps that we are trying to pursue,” British Security Minister Ben Wallace said.
“I’m waiting for the phone call from the Russian state.”
Kremlin spokesman Dmitry Peskov said he did not know who Ben Wallace was but said Russia had offered Britain its assistance in investigating the nerve agent attack and had been rebuffed.