The Russian state being responsible for the Salisbury spy attack "is a reasonable conclusion", the First Minister has said.

Nicola Sturgeon backed the Prime Minister's stance on the attack during the weekly session of First Minister's Questions.

British security experts believe the nerve agent used in the alleged attempted murder of Sergei Skirpal, a former Russian spy who acted as a double agent for MI6, and his daughter, Yulia Skripal was a Soviet-era chemical called Novichok.

On Wednesday, May expelled 23 Russian diplomats who were identified as "undeclared intelligence officers" after Moscow to provide an explain for the nerve agent's deployment in Britain.

Scottish Conservative leader Ruth Davidson quizzed Sturgeon on her views on the attack and on the Kremlin-funded broadcaster Russia Today at Holyrood.

The Conservative MSP asked Sturgeon if she would "join me in welcoming these robust and proportionate measures" against Russia.

"In a democracy, of course, people are right to ask questions and scrutinise the evidence," answered Sturgeon.

"Of course this investigation is on going and that will continue to be the case.

"But based on what I have been told, in my case that includes briefings directly from the national security advisor, I believe at this stage the conclusion that Russia was responsible is a reasonable conclusion.

"Therefore it demands a proportionate but a very firm response."

Sturgeon's comments came shortly before a rare joint statement by the Prime Minister alongside the leaders of the United States, Germany and France stating Russian responsibility was the "only plausible explanation".