It said the Salisbury attack came amid “a pattern of earlier irresponsible Russian behaviour”. This message was reinforced hours later as the US increased sanctions over Russian interference in the 2016 elections, and accused Russia of a cyber-attack against its energy grid and other infrastructure.
US officials said malware had been found in the operating systems of several organisations and companies in the US energy, nuclear, water and “critical manufacturing” sectors, with the attacks allegedly traced to Moscow.

The statement – described by Downing Street as the first time the leaders of the four nations had spoken together in such a way – marked a toughening of the rhetoric against Moscow, which continued to insist it had nothing to do with the attack.

The leaders said they “abhorred” the Salisbury incident, and the involvement of the Russian-made novichok “constitutes the first offensive use of a nerve agent in Europe since the second world war”.

“It is an assault on UK sovereignty and any such use by a state party is a clear violation of the chemical weapons convention and a breach of international law,” their statement said.