(CNN) -- Football club Barcelona paid 13.5 million euros ($18.5 million) in additional taxes on Monday, while denying any fiscal "irregularities" in its purchase of Brazilian superstar Neymar, which is now under investigation by a court.
FC Barcelona announced the surprise payment on its website, which it called "voluntary," just four days after a judge at Spain's National Court in Madrid filed preliminary charges against the fan-owned club for alleged tax fraud in the Neymar deal.
"The board (of Barcelona) denies the existence of any tax-related crime in relation to the fiscal obligations arising from the signing of the player (Neymar)," the club's website said.
"Throughout the process, the club was receiving expert advice and at every moment the club's auditor was informed and had access to all the documentation concerning the negotiations."
Last Thursday, Judge Pablo Ruz expanded his investigation against the club, as requested a day earlier by a prosecutor, who alleged the club avoided paying 9 million euros (more than $12 million) in taxes on Neymar's signing and transfer, corresponding to contracts signed in 2011 and 2013.
The judge is already investigating whether a former Barcelona president committed financial irregularities in the signing of Neymar.
The club said on its website Monday: "Given the existence of a possible divergent interpretation of the exact amount of tax responsibility arising from the signing, and to defend the club's reputation and good name, FC Barcelona has this morning made a complementary tax declaration of a total of 13,550,830.56 euros."
The reason, the club added, was "to cover any potential interpretation made concerning the contracts signed in the transfer process for Neymar, although we remain convinced that the original tax payment was in line with our fiscal obligations."
Judge Ruz said in his writ last Thursday that the investigation would include a look at an alleged "contract simulation presumably carried out by the parties which signed the contracts."
The expanded investigation would hold FC Barcelona responsible as an entity or business enterprise if found guilty of tax fraud, said a National Court spokesman, who by custom is not identified.
The judge ordered Spain's tax collection agency to examine FC Barcelona's tax filings for 2011, 2012 and 2013, the writ said.
Neymar joined Barcelona in June 2013, with his father -- who doubles as his agent -- having entered into an initial contract with the Spanish giants in late 2011.
Barcelona's former president, Sandro Rosell, who resigned last month after a judge agreed to investigate a lawsuit against him for alleged irregularities in the Neymar deal, has denied any wrongdoing.
Ruz began the investigation last month after a legal complaint from one of the club's members.
Prosecutor Jose Perals last Wednesday formally asked the judge to expand the investigation.
In his writ, Perals alleges that he's "become aware of a series of economic transactions by the club, and in favor of various companies, related to the signing of Neymar da Silva Santos Jr., based at times on simulated contracts."
The investigation originally came after season-ticket holder Jordi Cases complained that the amount paid to bring Neymar from Brazilian side Santos was more than the reported fee of 57.1 million euros.
A day after Rosell resigned, the club released figures that showed that Barca actually paid 86.2 million euros for Neymar.
Cases' lawyer, Felipe Izquierdo, earlier told CNN that other Barcelona executives, still working at the club after Rosell's resignation, also allegedly signed some of the contracts that brought Neymar to the club.
Barcelona has told CNN that one of those signatories was former club VP, and now President, Josep Maria Bartomeu.
Piers Edwards and Chris Murphy contributed to this report.