SALISBURY, England (Reuters) - Yulia Skripal has left hospital more than five weeks after she and her father, a former Russian spy, were poisoned with a nerve agent in an attack that has sparked one of the biggest crises in the West’s relations with the Kremlin since the Cold War.

Yulia and Sergei Skripal, 66, a former colonel in Russian military intelligence who betrayed dozens of agents to Britain’s MI6 foreign spy service, were found unconscious on a public bench in the English cathedral city of Salisbury on March 4.

Britain accused Russia of being behind the nerve agent attack and Western governments including the United States expelled over 100 Russian diplomats. Russia has denied any involvement in the poisoning and retaliated in kind.

The Skripals were in a critical condition for weeks and doctors at one point feared, even if they survived, they might have suffered brain damage. But the Skripals’ health since then has begun to improve rapidly.

Yulia, 33, has been discharged from Salisbury District Hospital, Christine Blanshard, medical director of the hospital, told reporters on Tuesday and her father could be discharged in due course.

“We have now discharged Yulia,” Blanshard said. “This is not the end of her treatment, but marks a significant milestone.”

“Her father has also made good progress,” Blanshard said. ” “Although he is recovering more slowly than Yulia, we hope that he too will be able to leave hospital in due course.”

Yulia has been taken to a secure location, the BBC said. The Sunday Times reported that Britain was considering giving the Skripals new identities and a fresh life in the United States to protect them from further attacks.

Russia said it would consider any secret resettlement of the Skripals as an abduction of its citizens.

Salisbury District Hospital is seen after Yulia Skripal was discharged, in Salisbury, Britain, April 10, 2018. REUTERS/Peter Nicholls

“The world, while having no opportunity to interact with them, will have every reason to see this as an abduction of the two Russian nationals or at least as their isolation,” a spokesman for the Russian embassy in London said.

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British Prime Minister Theresa May said the Skripals were poisoned with Novichok, a deadly group of nerve agents developed by the Soviet military in the 1970s and 1980s.

Russia has denied Britain’s charges of involvement in the first known offensive use of such a nerve agent on European soil since World War Two and suggested Britain carried out the attack itself to stoke anti-Russian hysteria.

Russia has said it does not have such nerve agents and President Vladimir Putin said it was nonsense to think that Moscow would have poisoned Skripal and his daughter.

Blanshard, a doctor with 25 years experience, said nerve agents work by attaching themselves to particular enzymes in the body that then stop the nerves from functioning. She said this had resulted in sickness and hallucinations.

Giving the first details about the medical treatment of the Skripals, she said doctors had first sought to stabilize them to ensure that they could breathe and that blood could circulate.

“We then needed to use a variety of different drugs to support the patients, until they could create more enzymes to replace those affected by the poisoning,” Blanshard said. “We also used specialized decontamination techniques to remove any residual toxins.”

She did not say when Yulia had been discharged.

Both Britain and Russia congratulated Skripal, a Russian citizen, on her recovery.

“We congratulate Yulia Skripal on her recovery,” the Russian embassy in London said, adding that it had not been granted consular access to her.

Sergei Skripal, who was recruited by Britain’s MI6, was arrested for treason in Moscow in 2004. He ended up in Britain after being swapped in 2010 for Russian spies caught in the United States.

Since emerging from the John le Carre world of high espionage and betrayal, Skripal lived modestly in Salisbury and kept out of the spotlight until he was found poisoned. He has British citizenship.