The intimate relations in one case lasted nearly a year, according to the alternative publication based in the Catalan capital.
Six women have filed a complaint against the officer, accusing him of sexual abuse. They argue their sexual consent was obtained on the basis of lies.
One of the women's lawyers, Mireia Salazar, told AFP the goal of the complaint was "to know how far these practices go, which in our opinion, have no legal justification."
The scandal deepened after the Madrid branch of climate activist group Extinction Rebellion said last week it had been infiltrated by a female police officer who "had sexual relations with at least one of its members".
The affair recalls the case in Britain of Kate Wilson, an environmental activist who was tricked into a sexual relationship with an undercover officer for nearly two years.
In a landmark ruling in 2021, a tribunal concluded that the police had violated her human rights.
In Spain the Hernandez case has sparked outrage, especially in the northeastern region of Catalonia which sparked the country's worst political crisis in decades in 2017 with a failed independence push.
It comes after Spain's central government admitted last year that it spied on the mobile phones of 18 Catalan separatist leaders using Israeli spyware Pegasus.
"Where is your moral limit, where is your ethical limit?" Gabriel Rufian, a top lawmaker with Catalan separatist party ERC, asked Spanish Prime Minister Pedro Sanchez last month during a debate in the assembly.
"It is not just a threat to political freedoms, ideological freedoms, but also -- it seems -- sexual freedoms", he added in a reference to the case of the undercover Barcelona police officer.
Sanchez's minority leftist coalition government regularly relies on the ERC to pass legislation in parliament.
Criticism has also come from far-left party Podemos, the junior coalition partners of Sanchez's Socialists.
"It is violence against women," secretary of state of equality, Angela Rodriguez of Podemos, told Catalan radio station Rac1.
"And I think that the sooner that we know what happened and justice can be done, the better it will be for the reputation of security agencies," she added.
'It was a shock'
The scandal comes as Sanchez's government grapples with waning support ahead of regional elections in May and a year-end general election.
Contacted by AFP, both the interior ministry and the police declined to comment on the allegations.
But during a recent debate in parliament, Interior Minister Fernando Grande-Marlaska dismissed the ERC's accusations of "illegal activities" by police as "a lie".
Undercover police also reportedly infiltrated a far-left group in the Mediterranean port of Valencia, and a Barcelona housing rights group called "Resistim al Gotic", although in these cases there are no allegations of improper sexual relations.
According to La Directa, a police officer calling himself Marc Hernandez pretended to be a "Resistim al Gotic" activist for nearly two years before the publication unmasked him in June.
"When the information was revealed, it was a shock," Marti Cuso, a member of the group, told AFP.
"We did not suspect anything, we had no clues that his person could be a police officer," he added.
© 2023 AFP