Britain, the United States, France and Germany say they are united in blaming Russia for the nerve agent attack on Sergei Skripal and his daughter. In a joint statement, the leaders say there is no "plausible alternative explanation" to the incident.

The UK has expelled 23 Russian diplomats and suspended high-level contacts with Moscow. Russia is expected to retaliate in similar fashion.

Al Jazeera's Barnaby Phillips reports from London.