Dr Christine Blanshard, medical director at the hospital, said: "I'm pleased to be able to report an improvement in the condition of Yulia Skripal. She has responded well to treatment but continues to receive expert clinical care 24 hours a day."
The Skripals' relatives in Russia had expressed fears that she was close to death and may even have already died.
Her improvement will be welcomed both by her family and detectives, who will hope that she recovers to the extent that she can respond in some way to questions about the hours leading up to the attack.
Yulia Skripal, 33, who lives in Moscow, was in Britain visiting her father and it may be that she can at some stage give an indication of whether they were followed or if she noticed any suspicious activity around her father's Salisbury house, where police say the largest concentration of traces of nerve agent was discovered.
David Videcette, a former counter-terrorism officer, said: "Officers will be hoping Yulia might be able to say something that indicates they were followed or that there was activity at the front of the house."
Detectives earlier said the pair were probably poisoned by a nerve agent being applied to Mr Skripal's front door at his home.
Britain has accused Russia of being behind the poisoning, something fiercely denied by the Kremlin. In turn, Russia has suggested that British intelligence officers may have been involved in the poisoning.