Citing unspecified documents and statements, the paper said Undercover Global Ltd, which was responsible for security at the embassy while Assange was staying there, sent the US intelligence service audio and video files of meetings he had with his lawyers.
The reports were allegedly handed over by David Morales, who owns the company and is currently being investigated by Spain's National Court, the paper said.
Speaking to AFP, one of Assange's lawyers confirmed the National Court was looking into the matter.
“There is a criminal case under investigation at the National Court but it is being conducted in secret… and we cannot say anything about what is being investigated beyond what has been leaked” to the press, Aitor Martinez told AFP.
The leak “probably came from employees at the firm”, he said.
Contacted by AFP, the court refused to comment.
According to El Pais, Undercover Global installed microphones in the embassy's fire extinguishers as well as in the women's toilets where Assange's lawyers used to meet for fear of being spied on.
'Live streaming to the CIA'
It said the company also installed a streaming system so the recordings could be directly accessed by US officials, enabling them to spy on a meeting Assange had with Ecuador's secret service chief Rommy Vallejo in December 2017.
At the time, they were planning to smuggle Assange out of the embassy and take him to another country by means of a diplomatic passport — but the plan never materialised.
At the end of April, Assange's lawyers filed an extortion suit against a group of Spanish nationals who reportedly used videos and documents from inside the embassy.
A source on his legal team said at the time Assange may have been targeted by “espionage”.
The Australian took refuge inside the Ecuadoran embassy in 2012, fleeing what he claimed was a politically-motivated extradition order for his return to Sweden for questioning in a sexual assault case.
He stayed there for seven years until April when he was dragged out of the embassy by the British police after Quito gave him up.
The 48-year-old is currently serving 50 weeks behind bars at a top-security prison in Britain for breaching his bail conditions when he fled into the embassy.