LONDON (AP) — Britain's leaders are trying to build international support as they respond to the poisoning of an ex-spy and wait for the Russian government's response to its claim that Russia was involved.

Officials said Tuesday Prime Minister Theresa May is reviewing a range of economic and diplomatic measures in retaliation for the assault with a military-grade nerve agent.

May has said it is "highly likely" Russia was involved in the poisoning of 66-year-old Sergei Skripal and his 33-year-old daughter, Yulia. Both remain in critical condition.

Her conclusion, based on assessment from the police and intelligence services, is leading to a major confrontation between Britain and Russia, which has taken an increasingly aggressive posture toward Europe in recent years.

The prime minister says Russia has until the end of Tuesday to explain its actions in the case.

Foreign Secretary Boris Johnson said Tuesday that Britain is talking to its international partners about the situation.

"I've been encouraged by the willingness of our friends to show support and solidarity," he said.

"I think in particular from President (Emmanuel) Macron of France, I talked to Sigmar Gabriel my German counterpart, and from Washington where Rex Tillerson last night made it absolutely clear that he sees this as part of a pattern of disruptive behavior ... malign behaviour by Russia ... the support for the reckless use of chemical weapons which stretches from Syria now to the streets of Salisbury."

Skripal, a former Russian military intelligence officer, was convicted of spying for Britain and then released in a spy swap. He had been living under his own name in the small city of Salisbury for eight years before the attack without attracting any public attention.

Former British ambassador to Russia Tony Brenton told Sky News May's plans must be more robust than the series of measures put in place when Britain was dealing with the Russian poisoning of former spy Alexander Litvinenko, who died after drinking radioactive tea in 2006.

At that time, Brenton said, Britain decided to use a package of measures designed to prevent such a killing from happening again.

"Obviously, it wasn't enough," he said.

He said economic and diplomatic measures would be used along with a possible lowering of Britain's official representation at the upcoming World Cup soccer tournament in Russia this summer.