Britain will expel 23 Russian diplomats after Moscow refused to explain how a Russian-made nerve agent was used in an assassination attempt on a former spy on UK soil.

Prime Minister Theresa May on Thursday morning (AEDT) described the diplomats, who have a week to leave the United Kingdom, as “undeclared intelligence officers”.

Former Russian double agent Sergei Skripal, 66, and his daughter Yulia, 33, were found unconscious on a bench in the city of Salisbury on March 4 and remain in hospital in a critical condition.

A police officer who helped the pair was also harmed and remains in a serious condition.

Ms May called the attack an unlawful use of force by Russia against the UK and said Britain would also introduce a range of new measures to strengthen defences against hostile state activities and freeze Russian state assets wherever there was evidence of a threat.

She also says no ministers or members of the British royal family would attend the soccer World Cup in Russia.

Russia, which denies any involvement in the attack, has called the measures announced by Ms May “unacceptable, unjustified and short-sighted” and warned Britain to expect retaliation.

Ms May has said the Skripals were attacked with Novichok, a Soviet-era military-grade nerve agent.

She had asked Moscow to explain whether it was responsible for the attack or had lost control of stocks of the highly dangerous substance.

“Their response demonstrated complete disdain for the gravity of these events,” Ms May said in a statement to parliament on Wednesday.

“They have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance.

“There is no alternative conclusion, other than that the Russian state was culpable for the attempted murder of Mr Skripal and his daughter, and for threatening the lives of other British citizens in Salisbury, including Detective Sergeant Nick Bailey.

“This represents an unlawful use of force by the Russian state against the United Kingdom.”

Ms May said the expulsion of the 23 diplomats, identified as undeclared intelligence officers, was the biggest single expulsion for more than 30 years and would degrade Russian intelligence capabilities in Britain for years to come.

The expelled Russian diplomats have one week to leave Britain, she said, before listing other measures.

“We will freeze Russian state assets wherever we have the evidence that they may be used to threaten the life or property of UK nationals or residents,” Ms May said.

She also said new legislative proposals would be urgently developed to counter any threat from a hostile state.

“This will include the addition of a targeted power to detain those suspected of hostile state activity at the UK border.”

British authorities would make use of existing powers to enhance efforts to monitor and track the intentions of those travelling to the UK who could be engaged in activities that represented a security threat.

“We will increase checks on private flights, customs and freight,” Ms May said.

She also threatened action against those she described as “serious criminals and corrupt elites”, adding: “There is no place for these people, or their money, in our country.”

Ms May said Britain would revoke an invitation to Russian Foreign Minister Sergei Lavrov to visit the country and suspend all planned high level bilateral contacts between London and Moscow.

Britain was not alone in confronting Russian aggression, Ms May said, adding she had discussed the Salisbury attack with US President Donald Trump, German Chancellor Angela Merkel and French President Emmanuel Macron.

US ambassador to the United Nations Nikki Haley said the US believes Russia was responsible for the attempted assassination Mr Skripal and his daughter, and the UN Security Council should hold the Kremlin “accountable”.

“The United States believes that Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent,” Ms Haley said at a UN Security Council meeting in New York.

“If we don’t take immediate, concrete measures to address this now, Salisbury will not be the last place we see chemical weapons used,” she said.

Britain asked for an urgent meeting of the United Nations Security Council to update members on the attack.

Foreign Minister Julie Bishop said Australia will consider upgrading sanctions on Russia once the UK completes its investigation into the attack.

Ms Bishop has spoken to her UK counterpart Boris Johnson about the issue.

“This attack is the use of chemical weapons in a civilian environment and Australia condemns, absolutely, any use of chemical weapons by anyone anywhere,” she told reporters on Wednesday.

Donald Tusk, European Union council president, voiced support for Britain and stood ready to put the attack on the agenda of a council meeting next week.

Ms May also said London had notified the Organisation for the Prohibition of Chemical Weapons about the use of the nerve agent.

-with AAP