Corrina Sayn-Wittgenstein, a German princess through a former marriage, has told Spanish media in recent weeks that she carried out "delicate" missions for the Madrid government.
The authority has, however, denied her claims.
Sayn-Wittgenstein, who runs a consulting firm and has called Juan Carlos "a national treasure", mediated between Madrid and Abu Dhabi to try to calm investor anger over Spain's decision to slash subsidies for renewable energy projects, the newspaper El Mundo reported.
Spanish Foreign Minister Jose Manuel Garcia-Margallo has not denied meeting with the German aristocrat, but he has "firmly" denied that Madrid ever relied on Sayn-Wittgenstein to mediate on behalf of the government.
The head of Spain's National Intelligence Agency, Felix Sanz Roldan, refused to comment on the claims made by Sayn-Wittgenstein as he arrived at the parliament to testify behind closed doors before a commission on state secrets.
"There is no comment to make because it is a commission on secrets," he told reporters.
But the leader of the tiny United Left party, Cayo Lara, who attended the proceedings said Roldan was "sincere" in his replies to lawmakers' questions.
He added, however, that many points still needed to be cleared up "regarding the action of the government as well as of the royal palace".
"I don't know if Corrina lies or not, but she has said many things in the media and they are very serious things for the state," he said.
"The state should respond and take measures, and when I speak of the state I mean the government as well as the head of state," he added in a reference to the Spanish monarch.
Sayn-Wittgenstein took part in a controversial elephant-hunting trip which Juan Carlos took in Africa last year during a time of record unemployment in Spain.
The 75-year-old king issued an unprecedented public apology after the trip, which threw the spotlight on his relationship with Sayn-Wittgentein. Her name has also become embroiled in a corruption scandal involving the monarch's son-in-law Inaki Urdangarin.
Last month she told El Mundo that she secured a job for Urdangarin, who is under investigation for allegedly embezzling millions of euros of public money, at the Laureus sports foundation in 2004 at the request of the king.
But she said Urdangarin, who is married to the king's youngest daughter, eventually decided not to accept the job.