Julian Assange has expressed shock after new documents revealed fresh details about the involvement of UK authorities in the long-running saga that has seen the WikiLeaks founder live inside the Ecuadorian embassy in London for the past three years.

He was granted political asylum after fighting extradition to Sweden where he faced sex allegations, which he has always denied. Assange fears that if he goes to Sweden he will be taken to the US for questioning over the activities of WikiLeaks.

He has offered to be interviewed inside the embassy in London but attempts to set up a meeting have foundered.

Emails obtained by Italian news magazine L’Espresso under the Freedom of Information Act showed that Britain’s Crown Prosecution Service wrote to the Swedish authorities in 2011, saying it would “not be prudent” for them to interview Assange in the UK.

Members of the media outside the Ecuadorian embassy in central London. Photograph: Peter Nicholls/Reuters

“Any attempt to interview him under strict Swedish law would invariably be fraught with problems,” said one email, dated 25 January 2011. Another email dated 13 January 2011 said: “Please do not think that the case is being dealt with as just another extradition request.”

Assange said: “This is astonishing. I have been on the phone with my lawyers all morning and they are as shocked as I am.”