Britain's security minister 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 poisoned with the same substance.

The two Britons are critically ill after what is thought to be a chance encounter with the poison after the March attack on former Russian agent Sergei Skripal and his daughter Yulia in Salisbury in March.

Britain has accused Russia of the poisoning of the Skripals, a charge Russia has repeatedly denied.

"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," Security Minister Ben Wallace told BBC radio on Thursday.

"They (Russia) are the ones who could fill in all the clues to keep people safe."