The evidence of the chief prosecution witness in the trial of British businessman Shrien Dewani for the murder of his honeymoon bride Anni was "riddled with contradictions", according to the trial judge.

Judge Jeanette Traverso made the comment during a ruling in Cape Town on an application by Dewani's defence lawyer Francois van Zyl to dismiss the prosecution case.

Anni Dewani was gunned down in Cape Town in November 2010 after the taxi she was travelling in with her husband was hijacked as they passed through a township late at night.

Three men have already been convicted of their role in the killing.

Prosecutors say bisexual Dewani wanted out of the relationship and plotted for his engineer wife, 28, to be killed in an apparent botched hijacking.

But Dewani, 34, has always denied any involvement in the plot.

Dewani, from Westbury-on-Trym, near Bristol, was charged with murder, conspiracy to commit kidnapping, robbery with aggravating circumstances, kidnapping, and defeating the ends of justice.

His lawyers last month applied for Judge Traverso to dismiss the case - although they have refused to confirm if Dewani would be free to return to the UK immediately.

Last week the family of Mrs Dewani - previously known as Anni Hindocha - begged her accused widower to ''tell the world what happened the night she died'' with her brother Anish imploring him to take to the witness stand and tell his story for the first time.

Judge Traverso was scathing of chief prosecution witness, cab driver Zola Tongo, part of whose evidence she described as "highly improbable".

Dewani claims the couple were hijacked as Tongo was driving them through Gugulethu in his minibus. He says he was released unharmed and his wife was driven away. She was found shot dead in the abandoned minibus in Khayelitsha the next morning.

Tongo is already serving an 18-year jail term.

Judge Traverso said it was crucial for the state's case to prove that he entered into an agreement with others to have Anni killed in 2010. Failing that, there would be nothing linking him to the crimes.

She said a defendant was entitled to be discharged if there was no possibility of conviction unless he entered the witness box and incriminated himself.

Tongo was the only accomplice witness she said, adding that such evidence should be treated with "caution".

Tongo's version needed to be corroborated specifically where it implicated the accused.

Details such as where he picked up and dropped off Dewani and his wife did not provide corroboration for Tongo.

"It is what was said during these events which is an issue and for that there is only the version of Tongo."

She said the same applied to phone calls between Tongo and Dewani.

"This telephone communication does not in itself corroborate what was said during those calls, it merely confirms that communication took place."

Dewani shook his head when the charges were read out and stared ahead intently.

His wife's family were also in court to hear the ruling.

Mziwamadoda Qwabe and gunman Xolile Mngeni have been convicted along with Tongo for their parts in the murder, which sent shockwaves through Cape Town, which is heavily reliant on tourism.

Qwabe is part-way through a 25-year jail sentence. Mngeni was serving life for firing the shot that killed Mrs Dewani, but died in prison from a brain tumour.