Yulia Skripal's 66-year-old father Sergei remains in critical condition.
Salisbury NHS Trust, which oversees the hospital where the Skripals are being treated, said 33-year-old Yulia is "improving rapidly and is no longer in a critical condition. Her condition is now stable."
"She has responded well to treatment but continues to receive expert clinical care 24 hours a day," said Dr. Christine Blanshard, medical director at Salisbury District Hospital.
The Skripals were found unconscious in the English city of Salisbury on March 4, and British authorities say they were poisoned with a military-grade nerve agent.
Police say they were likely exposed to it on the door of Sergei Skripal's suburban house in Salisbury, where the highest concentration of the chemical has been found.
Sergei Skripal, a former Russian military intelligence officer, was imprisoned after he sold secrets to British intelligence. He was released in a 2010 spy swap and moved to Britain.
- New Cold War? What the future holds for Russia, its allies and its foes
- Moscow trolls Twitter for response after dozens of diplomats expelled by U.S., Europe
- Russia blames 'powerful forces' in U.S. and U.K. for chemical attack on spy
Britain says he and his daughter, who was visiting from Russia, were poisoned with a nerve agent developed in Soviet times and that it must have come from Russia.
Moscow vehemently denies involvement in the attack, which has sparked a diplomatic crisis between Russia and the West.
More than two dozen countries have expelled more than 150 Russian diplomats, including 60 kicked out by the U.S. The former Soviet republic of Georgia joined the list Thursday, saying it would expel one Russian diplomat in solidarity with Britain.
The announcement follows the expulsion of Russian diplomats by European Union nations, the U.S., NATO and other countries.
Georgia's Foreign Ministry said in a statement on Thursday that the diplomat has been declared persona non grata and must leave within a week. The ministry condemned the poisoning, calling it a "serious challenge to common security."
Georgia severed diplomatic ties with Russia following a brief war in the breakaway republic of South Ossetia. Russian diplomats have been operating out of the special interests section of the Swiss Embassy in the Georgian capital, Tbilisi, since 2009.