The Guardian quoted May as saying, "23 Russian diplomats will be expelled. They have been identified as undeclared intelligence officers. It will be the largest expulsion for 30 years."
The British Prime Minister further said that the diplomats will be given a week to leave the country.
Britain will also cancel high-level contacts with Russia, said May.
May said that they have withdrawn the invitation send to Russian Foreign Minister Sergei Lavrov to visit the country. It is pushing for a debate at the UN, she said.
Earlier on Tuesday, United States President Donald Trump backed the British Government's theory which holds Russia responsible for the poison attack that left a former Russian double agent and his daughter comatose on a park bench last week.
"It sounds to me like it would be Russia, based on all the evidence they have. It sounds to me like they believe it was Russia and I would certainly take that finding as fact," CNN reported Trump as telling reporters outside the White House.
Trump further underscored, "As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be."
In a strongly worded statement to the House of Commons, May said the "military grade" nerve agent used in the attack had been identified as Novichok, a substance developed in the Soviet Union in the 1970s.
In a move that is likely to plunge the UK-Russia ties to a new low, the British Prime Minister has demanded a response from the Russian government by the end of Tuesday, making it clear, "Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom."
Dismissing May's comments Russian Foreign Ministry spokeswoman Maria Zakharova, told CNN, "This is a circus show in the British Parliament. The conclusion is obvious."
Retired military intelligence officer Skripal, 66, and his daughter, Yulia, 33, were found slumped on a bench in Salisbury city centre on March 4. (ANI)