Lawyers representing Wikileaks founder Julian Assange have asked the High Court to block his extradition to Sweden on sexual assault allegations.
Ben Emmerson QC, said his arrest warrant was "flawed" as it failed to fairly describe the claims against him.
He said Mr Assange was a victim of a "mismatch" between English and Swedish law on what constituted a sex crime.
Mr Assange says the case is politically motivated, as his website has leaked a mass of sensitive US diplomatic cables.
His appeal challenges a ruling at Belmarsh Magistrates' Court in February, which said extradition would not breach his human rights.
At that court, District Judge Howard Riddle concluded Mr Assange would get a fair trial if he was ever charged.
The Australian computer expert, 40, is wanted for questioning on three allegations of sexual assault, including one of rape, said to have taken place in Stockholm in August last year.
The claims were made by two female Wikileaks supporters.
His lawyers say the claims against him are not extradition offences, and sending him to Sweden would be an abuse of process breaching his human rights.
Mr Emmerson told the court the extradition order was flawed as it was sought "not for prosecution but for the purposes of an investigation", which amounted to a "disproportionate" use of the European arrest warrant.
He said he was not questioning the credibility of the women who made the allegations, nor the "genuineness of their feelings of regret about having had consensual sex with Mr Assange".
He also said he was not challenging the fact they found his sexual behaviour "disreputable, discourteous, disturbing or even pushing towards the boundaries of what they were comfortable with".
Nevertheless, he went on, the sexual activities were consensual and could not be criminalised under English law.
Mr Assange fears extradition to Sweden may lead to him being sent to the US to face separate charges relating to Wikileaks, for which he could face the death penalty.
The Australian won bail in December and has been staying at Ellingham Hall, a 10-bedroom Norfolk farmhouse owned by Vaughan Smith, director of the Frontline media club.
His bail conditions include wearing an electronic tag and daily appearances at a nearby police station.
A small number of supporters had gathered outside the High Court as Mr Assange arrived.
Mr Assange held a lavish 40th birthday party at Ellingham Hall at the weekend, attended by celebrities and supporters.