London (CNN) The health of the daughter of a former Russian double agent poisoned in a nerve agent attack in the UK is "improving rapidly," according to the hospital treating her.

Sergei Skripal and his daughter Yulia, 33, were poisoned on March 4 after being exposed to what the British government says was a military-grade nerve agent. They had been hospitalized in a critical condition since the attack.

"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," Dr. Christine Blanshard, Medical Director for Salisbury District Hospital, said in a statement.

Sergei Skripal, 66, remains in a critical but stable condition but Yulia is "improving rapidly," the statement said.

The Skripals were found slumped on a bench in an outdoor shopping complex in Salisbury, England. They had no visible injuries, according to police.

Read More