Ecuador has accused one of the United Kingdom's leading private security and surveillance firms of bugging its London embassy where WikiLeaks publisher Julian Assange is lodged.

WikiLeaks has denounced the electronic espionage operation as an instance of "imperial arrogance."

"Aside from the gross violation of the integrity of Ecuador's embassy, no candidate in Australian election should be subject to covert surveillance," Mr Assange told Fairfax Media today.

Foreign Minister Ricardo Patiño has confirmed a sophisticated listening device was found inside the office of then Ecuadorian ambassador to the United Kingdom, Ana Alban, two days before the Foreign Minister visited the embassy to meet with Mr Assange on June 16.

Mr Patiño added that the Ecuadorian authorities "have reason to believe that the bugging was being carried out by the company, the Surveillance Group Limited, ... one of the biggest private investigation and undercover surveillance companies in the United Kingdom."