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May has laid culpability for the attack on Sergei Skripal and his daughter with Russia, though Russia denies involvement.
She was asked about the response of ex-US administration official Rex Tillerson to the incident. He had joined Britain in saying Russia was likely responsible for the poisoning, just hours before he lost his job as US Secretary of State.
"We are leaving our American colleagues in no doubt about the seriousness of this issue," May told parliament.
"Mr Tillerson did indeed give robust comments after this incident took place, and I am sure we will be working with his replacement to ensure that America is one of those allies who stand alongside us."