One of Spain's most prominent conservative unionists is facing resignation calls after being accused of trying to smear Catalan independence supporters.

Spanish Interior Jorge Fernández Díaz allegedly conspired with his country's anti-fraud office to seek dirt on senior members of the two main pro-independence parties in Catalonia.

Mr Fernández has been under growing pressure to step down ever since the claims were first revealed by a Madrid online newspaper on Tuesday - just days before he tries to defend his seat, in the Catalan capital Barcelona, in Spain's general election.

His ultra-unionist party, Partido Popular or PP, has been operating a minority caretaker government in Madrid ever since inconclusive elections in December.

How story is covered in Catalonia: "Dirty War in Plain Sight," splashes Ara

HeraldScotland:

The online paper which broke the story, Público, published extracts of conversations between Mr Fernández and the director of Spanish Anti-fraud Office in Catalonia, Daniel de Alfonso Laso.

Mr Fernández was urging Mr Laso to find suspicious dealings, business or family connections of officials and politicians in both the left-wing Esquerra Republicana, the formal ally of the SNP, and the liberal Convergència parties.

The conversations were alleged to have taken place in late 2014, just after the Scottish independence referendum when Catalan government officials were seeking to carry out their own plebiscite. That vote went ahead as a kind of protest action rather than an officially sanctioned referendum.

Mr Fernández has insisted that Público, which published audio of exchanges between the two men, had twisted the conversation. He said: "I am the victim of this outrage.

"This was a private conversation that has been produced two years after it took place and is now being published out of context in a fragmented, slanted, self-interested and leading way."

The politician has ordered an investigation in to the leak, which he said was "designed to hurt a political adversary".

Picture: Mariano Rajoy

HeraldScotland: Spanish Prime Minister Mariano Rajoy had previously insisted that his country's banking sector would not need a bailout (AP/Alberto Di Lolli)

The scandal comes amid increasingly bitter exchanges between pro-independence parties - or independentistes - in Catalonia and Spanish unionists.

Independentistes have latched on to the story as evidence that the Spanish and the PP in particularly use dirty tricks.

Esquerra’s General Secretary, Marta Rovira, argued that the Spanish Government “constantly plots against parties and democratic guarantees” and therefore called for Mr Fernández’s “immediate resignation”.

She said the scandal reminded reminds her of “The Godfather" film.

Catalan Government spokeswoman, Neus Munté, said that she was “outraged” by the news and said on Twitter that “urgent explanations” were needed, as well as “assuming responsibility”.

Carles Mundó, the Catalan minister for Justice, said: “We call for Mr Fernández's resignation and that of the Director of the Spanish Anti-fraud Office. It is unacceptable that such a meeting to plot against political adversaries took place".

Mr Laso, reacting to the reports on radio, said: "I will not resign, it is the Minister [for Home Affairs] who should do so."

Ahead of June 26 general elections, the most recent polls show support for independence ahead of staying in Spain but with many undecideds.

All main Spanish parties oppose an independence referendum for Catalonia. Podemos, the anti-austerity party, this week dropped its insistence on such a poll as a condition of support for a future coalition in Spain.

This could pave the way for an alliance between Podemos and the Socialists to remove Mr Fernandez and Mr Rajoy's PP from power. Socialist leader Pedro Sanchez had ruled out any coalition with Podemos that would lead to the break up of Spain.

Spain's main parties spent months bickering over coalition talks after December's general election. All, including Podemos, oppose independence.