Shrien Dewani's 28-year-old wife Anni, a Swede of Indian origin, was shot when a taxi the couple were travelling in was hijacked in November 2010. He is facing extradition to South Africa.
He was being treated in a secure mental health hospital for depression and post-traumatic stress disorder so he can be extradited to face the charges, BBC reported.
But his family said he had been moved after suffering chest pains.
They said he had been transferred to a different hospital after tests suggested he had reacted poorly to medication prescribed as part of his mental health treatment.
Dewani, 32, from Bristol denies plotting to kill his wife. His family said his bail conditions had been altered at a hearing at Westminster Magistrates' Court.
A statement from the Dewani family said: "Shrien's health has seriously deteriorated.
"He was admitted to hospital last weekend suffering from severe chest pains.
"He also has a raised creatine kinase level, indicative of another poor reaction to the medication he is being prescribed to improve his mental health.
"We are relieved that doctors have managed to stabilise him. We are extremely grateful for the swift action of the hospital staff and are hopeful of a recovery."
In March, the High Court ruled that it would be "unjust and oppressive" to extradite Dewani to South Africa, as his mental condition had deteriorated since his arrest.
His lawyers wanted proceedings to be halted for a year to give him time to recover.
But District Judge Howard Riddle has set 1 July 2013 as the date on which proceedings should continue.
Dewani faces charges of murder, conspiracy to commit murder, robbery with aggravating circumstances and obstructing the administration of justice - all of which he denies.
In November Xolile Mngeni, 25, was found guilty of Anni's murder by a judge in Cape Town. He was later given a life sentence.
Mngeni's co-accused have said they helped kill Dewani, on the orders of her husband.
London: An Indian-origin British businessman, accused of ordering the murder of his bride during their honeymoon in South Africa, has been admitted to hospital after his health "seriously deteriorated".
First Published: Thursday, February 14, 2013, 20:58