Image copyright Reuters Image caption Brock Turner was seen by two other students sexually assaulting the woman in 2016

A lawyer for a former US student convicted of sexually assaulting an unconscious woman has launched an appeal to have the ruling overturned.

Brock Turner was sentenced to six months in jail in 2016 for assaulting the woman outside a Stanford University fraternity house a year earlier.

The sentencing judge was widely criticised for leniency and removed from office by voters in June.

On Tuesday, his lawyer argued there had been insufficient evidence to convict.

Turner, a champion swimmer, was found guilty of three felony charges.

His lawyer Eric Multhaup told justices at a California appeals court that there was no evidence to prove at what point the woman became unconscious, US media report.

He said the jury had had to "speculate" and had "filled in the blanks" to reach their conclusion.

Mr Multhaup described his client's actions as "outercourse", meaning a sexual act while clothed, as opposed to intent to rape.

Deputy Attorney General Alisha Carlile argued that there had been "ample" evidence for the convictions.

The appeals court judges appeared sceptical of Mr Multhaup's arguments, the Associated Press news agency reported. They are expected to reach their decision within 90 days.

Turner, 22, was not present during Tuesday's hearing.

Although prosecutors had sought a six-year term, Turner was sentenced to six months and released after serving only three. He was placed on the sex-offenders register for life.

Image copyright Getty Images Image caption Anger at the sentence sparked a campaign to have Judge Aaron Persky recalled

The case sparked a debate in the US about sexual assault and whether white men from wealthy backgrounds are treated more favourably by courts.

Outrage at the sentencing was compounded by a letter from Turner's father saying his son's life would "never be the one that he dreamed about... a steep price to pay for 20 minutes of action out of his 20-plus years of life".

The victim, who has remained anonymous, directly addressed Turner in court and made a moving impact statement that was widely read online.

Judge Aaron Persky was later removed from his post by voters in Santa Clara County following a campaign to unseat him.

He said he had no regrets about the case and had been bound by sentencing and probation guidelines.

Judge Persky was cleared of misconduct by the California Commission on Judicial Performance.