BERLIN (Reuters) - A conservative ally of German Chancellor Angela Merkel said on Tuesday (March 13) that if Russia fails to cooperate in the investigation into a nerve agent attack on a former Russian double agent in Britain, there should be a joint Western response.

"The behaviour of the British government is rational," Norbert Roettgen, chairman of Germany's parliamentary foreign affairs committee, told Reuters.

"The discovery of a Russian military substance means that Russia cannot refuse cooperation in the clearing up of this matter," he said.

"If Russia does not cooperate, there must be a joint Western response."

Sergei Skripal, 66, and his daughter Yulia have been in hospital since March 4 when they were found unconscious in the English city of Salisbury.

British Prime Minister Theresa May said on Monday it was highly likely Moscow was to blame after Britain identified the substance as part of a group of nerve agents developed by the Soviet military in the 1970s and 1980s.

Asked about possible sanctions against Russia, Roettgen said any measures should target the fudged boundary between the Russian state, security apparatus and organised crime.

"This case should be reason for Britain to examine its open stance towards Russian capital of dubious origin," he said.