The family of the British businessman accused of arranging his wife's murder during their honeymoon in South Africa say they are concerned over his failing mental health.

Shrien Dewani is described as increasingly fragile and friends say he has lost 10kg in weight during recent weeks and is tormented by flashbacks of the hijack, during which his wife Anni, 28, was taken and shot near Cape Town.

The newlyweds' taxi was hijacked in the Gugulethu township on 13 November. The driver and Dewani were left at the roadside and his wife was later found dead in the back of the abandoned cab with a bullet wound to her neck.

Four South Africans have been arrested in connection with the murder and one has been sentenced to 18 years in jail, but the country's police insist the plot was "masterminded" by Dewani and have called for his extradition.

A close family friend said that while the task of clearing his name was keeping the 30-year-old going, there was concern that his depression might become too much. He said: "This is a guy who is completely devastated, a shadow of his former self. Anybody who has seen him is shocked by his appearance. You have to remember this is someone who has been widowed at the age of 30, two weeks after he was married.

"He's fragile and unable to sleep because of the recurring flashbacks of having a gun put to his head and then his wife is driven off. As a family even we still can't fully comprehend what must be going on his mind. He is taking therapy but we are worried."

Dewani is said by his family to have been diagnosed with post-traumatic stress disorder by a psychiatrist and last week was unable to attend an extradition hearing at City of Westminster magistrates court. However, his family say the businessman is determined to make a full recovery and might voluntarily return to South Africa to fight the allegations against him.

"He's doing everything he can to get back on track so that he can clear his name. Post-traumatic stress disorder doesn't go away in a matter of days but the intention is to fight the allegations. No one would want to stay on bail under these conditions," said the friend.

Dewani, who owns a care home business in Bristol, where he lives, has been unable to work since his wife died.

His family are keen to seek assurances that if he returns to South Africa to stand trial, he will not be mistreated.

"He is going to go back into an extremely hostile environment. He's hardly Mr Popular out there and it is reasonable to secure assurances that there would not be an opportunity for a plastic bag to be put over his head or a torch shone into his face until the right answer was given," he added. The family say they are confident that once the facts behind the case emerge Dewani will be cleared by a South African judge.

In a plea bargain with the South African authorities, the taxi driver, Zola Tongo, alleged that Dewani offered him 15,000 rand (£1,400) for the killing. Tongo, 31, of Cape Town, was sentenced to 18 years in jail for murder, kidnapping, robbery with aggravating circumstances and perverting the course of justice.

Dewani's family say there is no evidence of a connection between Tongo and Dewani and that most aspects of the case are not supported by proof.

South African police commissioner Bheki Cele has claimed that he has finally established a motive behind the murder of Dewani's bride and promised that the details would be disclosed at Thursday's extradition hearing. However, the promised disclosure never emerged after Dewani failed to attend court.

The senior South African investigating officer on the case also claimed last week that he had gathered further "good leads" since arriving in London.

Dewani may be able to defer his appearance in court a number of times, with one expert warning last night that it could take up to 12 months before a final decision is reached. Extradition specialist Karen Todner said: "It's all subject to him being able to face proceedings. It's always difficult to fight an extradition case because you start with the basis that we have an agreement with South Africa. But since 2007 South Africa does not have to produce any evidence."