Britain's Foreign Secretary says Russia has been stockpiling the deadly nerve agent used to poison a former double agent in England.

Boris Johnson also claims it's been investigating how such weapons can be used in assassinations.

Russia continues to deny any involvement in the attack of former double agent Sergei Skripal and his daughter Yulia in Salisbury.

UK correspondent Gavin Grey says Russia's EU ambassador is trying to spread the blame.

Mr Gray says he's "suggesting that nerve agent could have come from the Czech Republic, Slovakia, Sweden, the United States, or the UK."

Britain and Russia have each expelled 23 diplomats over the attack, as relations between the two countries crash to a post-Cold War low.

LISTEN ABOVE AS GAVIN GREY SPEAKS WITH KATE HAWKESBY