Julian Assange can be extradited to the US, the high court has ruled as it overturned a judgment earlier this year.

The decision deals a major blow to the WikiLeaks co-founder’s efforts to prevent his extradition to the US to face espionage charges, although his fiancee immediately indicated that an appeal would be launched.

The senior judges found that a then-district judge had based her decision earlier this year on the risk of Assange being held in highly restrictive prison conditions if extradited.

But in their ruling on Friday, they sided with the US authorities after a near-unprecedented package of assurances were put forward that Assange would not face those strictest measures either pre-trial or post-conviction unless he committed an act in the future that required them.

Lord Burnett said: “That risk is in our judgment excluded by the assurances which are offered. It follows that we are satisfied that, if the assurances had been before the judge, she would have answered the relevant question differently.”