LONDON, March 7 (Reuters) - Russian double-agent Sergei Skripal was poisoned by a very rare nerve agent, a reporter from the Sun newspaper said on Wednesday ahead of a police briefing on the case.

“Sergei Skripal was poisoned by a very rare nerve agent, which only a few laboratories in the world could have produced - security sources confirm,” the Sun’s political editor, Tom Newton Dunn, said on Twitter.

British police are shortly due to hold a media briefing about Skripal, who earlier on Wednesday was described as being in a critical condition in hospital after falling ill in the southern English city of Salisbury on Sunday. (Reporting by David Milliken and Alistair Smout; editing by Stephen Addison)