A military-grade nerve toxin attack on former Russian double agent Sergei Skripal and his daughter may have left them with compromised mental capacity and it is unclear whether they will recover, a British judge said on Thursday, Reuters reported.

Britain has said Russia used a Soviet-era chemical weapon called Novichok to attack Skripal and his daughter Yulia in the first known offensive use of such a weapon on European soil since World War Two. Russia has denied any involvement.

“The precise effect of their exposure on their long term health remains unclear, albeit medical tests indicate that their mental capacity might be compromised to an unknown and so far unascertained degree,” Judge David Williams said in his ruling.

They were both in a physically stable condition and were being treated “on the basis they would wish to be kept alive”, the doctor said, according to Williams’s ruling at the London’s Court of Protection, which makes decisions about the welfare of people who are unable to do so themselves.