The UK could take further measures against Russia, such as closing UK waters to Russian vessels, following the nerve agent attack in Salisbury, but must stop short of the use of military force, a legal expert has said.
Professor Michael Schmitt, an expert in the legality of the use of military force and cyber warfare, said: "The Prime Minister has labelled the assassination attempts a 'use of force'.
"Under international law, the use of force by one state against another is a violation of the UN Charter and customary international law unless it is done in self-defence or with Security Council authorisation.
"But the use of force by the UK in self-defence is not authorised; a military response by the UK would be precluded by the principle of 'immediacy', which does not allow a military response against an attack that is over.
"International law does not permit a victim state to simply retaliate against or punish another state.
"As the Russian operation violated international law, the UK is entitled to take what in international law are called 'countermeasures', such as closing UK territorial waters completely to Russian flagged vessels until, for instance, Russia assured the UK there would be no further such actions, compensated the individuals affected, or apologised.
"Such a measure would normally be unlawful, but the fact that it is intended to achieve such purposes makes it lawful."
Prof Schmitt, who is professor of public international law at the University of Exeter, added: "What has been done - expelling diplomats - is known as retorsion in international law.
"This term refers to an action that is 'unfriendly' but is not unlawful and therefore does not require an unlawful act by another country to justify it."