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Wikileaks founder Julian Assange has launched an appeal against extradition to Sweden arguing the arrest warrant is "invalid and unenforceable".

His lawyers told the Supreme Court judges the Swedish prosecutor who issued his European Arrest Warrant did not have the authority to do so.

Mr Assange is wanted by the Swedish authorities for questioning over alleged sex offences, which he denies.

Judgement is expected to be reserved to a later date.

The 40-year-old Australian, who remains on conditional bail in the UK, claims the allegations against him are politically motivated.

He is accused of raping one woman and "sexually molesting and coercing" another in Stockholm in August 2010.

Mr Assange's Wikileaks website published a mass of material from leaked diplomatic cables embarrassing several governments.

The key legal question for the seven judges is whether the prosecutor who issued the arrest warrant had the judicial authority to do so under provisions of the 2003 Extradition Act.

Invalid

Mr Assange's lawyer, Dinah Rose QC, said it was "a matter of fundamental legal principle" that the person issuing such a warrant was both impartial and independent of the parties in the case.

But she said the Swedish prosecutor was a party in the Assange case and therefore was not either of these things.

Ms Rose submitted: "Since the Swedish prosecutor cannot fulfil those conditions, she is not a judicial authority and not capable of issuing a warrant for the purposes of the 2003 Act."

She was breaching the principle that "no-one may be a judge in their own cause", Ms Rose added.

In the UK, judges can issue arrest warrants, and courts honour warrants issued by "judicial authorities".

The High Court, which previously approved his extradition, had recognised that the status of the public prosecutor was debatable.

But Ms Rose said it had "nonetheless concluded that the Swedish prosecutor was a 'judicial' authority within the meaning of Part 1 of the 2003 Act" and was "wrong to reach that conclusion".

Draconian instrument

Ms Rose said Swedish prosecutors could investigate without Mr Assange being extradited using telephone, by video-link or in person at an embassy.

She said: "The EAW is a draconian instrument which affects individual liberty, freedom of movement and private life: it should only be resorted to if other, less invasive, measures for achieving the general interest have failed or are unavailable."

But Clare Montgomery QC, for the Swedish Prosecution Authority, argued that the High Court was plainly correct to accept that the term "judicial authority" had a wide meaning.

She said when the EAW framework was set up, the drafters had intended it to include the prosecutors of many countries.

This "broader approach" recognised the "historic role" of public prosecutors within EU member states in authorising arrests and making extradition requests, she said.

Supporters gathered outside the Supreme Court to greet Mr Assange as he arrived for the hearing.