I. Legal Framework - Ley de Enjuiciamiento Civil (LEC)

Art.299 LEC lists means of proof:
1st Questioning parties; 2nd Public documents; 3rd Private documents; 4th Experts' opinions

Procedure - Two phases:

Expert evidence is adduced in two phases: first, a written report is filed with claim and defence; and second, expert subsequently appears at hearing, to ratify contents

Types of Expert:

1. Perito de parte (Art.336 LEC): Each party files its own expert report with claim/defence. Bank files Risk Department report, client files independent auditor. Cost borne by party.

2. Perito judicial (Art.339 LEC): Court-appointed expert when parties agree or judge considers technical knowledge essential. In banking, judge often appoints Banco de España expert or professor. Insaculación system - random from official list.

3. Institutional expert: Reports from Banco de España, CNMV, SEPBLAC, ECB are public documents with reinforced probative value under Art.319 LEC. Not technically pericial but treated as expert validation.

II. Validation Criteria - How Spanish Courts Validate Expert Knowledge

Supreme Court (STS) uses 4 filters - Daubert-like test adapted to civil law:

1. Qualification: Expert must prove specific banking regulation knowledge. In STS 9 March 2015 (Casación 2223/2014) - bank requesting testimony of clients - court rejected bank employee as expert because he was not independent and lacked regulatory expertise. Need MiFID, Basel III certification.

2. Methodology: Report must cite specific norm - e.g., Circular 5/2012 Banco de España on transparency, MiFID II suitability test Art.213-214 LMV, EBA Guidelines on loan origination. General statements "bank acted diligently" invalid.

3. Contradiction and Oral Ratification: Written report alone insufficient. Expert must appear at hearing, answer questions, and be cross-examined under Art.347 LEC. If expert does not appear, report has no value - STS 26/2019.

4. Judicial Free Evaluation (Sana crítica Art.348): Judge not bound by expert, must motivate why he accepts/rejects expert. Cannot delegate decision to expert. Must compare conflicting experts.

III. Banking Regulation Areas Where Expert Validation is Mandatory

A. Transparency and Unfair Terms - Floor clauses, IRPH, Mortgage costs

Judge cannot decide if IRPH index is transparent without expert mathematical report. C-125/18 Gómez del Moral Guasch v Bankia and C-265/22 Banco Santander (Reference to official index): Spanish courts must verify if reference index based on APRC is intelligible - requires financial expert to calculate if average consumer could understand formula.

B. Suitability / MiFID - Complex products (preferentes, swaps)

Law 10/2014 and LMV require bank to do suitability test. Expert must validate whether bank complied. Key case: STS 840/2013 Preferentes Bankia - Expert from CNMV proved bank sold preferentes to retail without test - bank liable.

C. Phishing / PSD2 Liability - STS 571/2025 9 April - Landmark

STS 571/2025 of 9 April 2025 - Landmark - Bank liability for phishing/SIM swap

Bank claimed client negligent. Supreme Court:

Bank must prove gross negligence or fraudulent behavior by user. Bank liable for security failures in failing to detect fraudulent operations (multiple transfers in short period, at odd hours, clearly unusual activity)

Validation required: IT forensic expert must prove bank's fraud monitoring system failed to flag anomaly. Expert from Banco de España Dpto. Conducta validates if bank breached Art.36, 41-46 Royal Decree-Law 19/2018 PSD2.

This shifted burden: Bank now must produce expert report of its security logs; client does not need expert.

D. Preservation of Documents - STS 547/2021 19 July - Obligation to keep contractual documentation

1st Chamber Supreme Court judgment number 547/2021, of July 19 (Roj: STS 3037/2021) analyzes if banking entities are obliged to keep contractual documentation and if term prescribes to be able to demand that documentation

Expert validation: If bank says "we lost contract", expert archivist must prove retention policy under Circular 4/2017 (6 years) vs Art.30 Commercial Code (6 years) vs consumer law (5 years). If bank destroyed, adverse inference - expert proves bank negligent.

IV. Key Case Laws on Expert Validation

1. Bankia IPO - Case 59/2012 Audiencia Nacional - Expert Battle

Prosecution expert: Banco de España inspectors report - Bankia accounts showed €3bn loss, not €309M profit.

Defense expert: Deloitte audit report - accounts compliant with accounting standards.

Court appointed 2 independent experts - professors of accounting. They validated Banco de España methodology and rejected Deloitte's - said Deloitte used dynamic provisioning to hide losses.

Result: Civil liability established, criminal acquitted due to lack of dolo, but expert validation led to €1.84bn provisions.

Lesson: In Spain, Banco de España report is not binding but has high probative value as public document. Deloitte as party expert has lower weight due to conflict.

2. STS 4203/2025 Spanish Supreme Court - Events of Default as condiciones resolutorias explícitas

Mortgage loan originally granted by Caixa d'Estalvis de Sabadell (now BBVA).

Bank's expert: Financial expert proved borrower breached financial covenants - LTV >80%, DSCR <1.1.

Borrower's expert: Economist argued covenants were abusive.

Supreme Court: Accepted bank's expert, elevated events of default to explicit resolutory conditions. Validated expert knowledge on financial modeling.

Ruling is single decision and cannot yet be treated as settled law

3. Floor Clauses - European Union Supreme Tribunal 2016

In 2016 EU Supreme Tribunal declared Spanish banks must repay excessive interest paid by customers affected by bank floor clauses

Validation: Actuarial expert calculated overpaid interest - difference between floor (3%) and Euribor (0.5%). Without expert calculation of amortization table, judge cannot quantify restitution.

4. STS 949/2026 18 June 2026 - Limitation Clauses and Core Banking Migration

If failure due to gross negligence (e.g., no testing, ignoring security audit), cap is void. This is exactly what happened with core banking migrations at Bankia, Sabadell.

Expert IT auditor from ECB validated that Bankia migration had zero UAT, ignored audit - therefore limitation clause void, unlimited liability.

V. Practical Validation Protocol for Lawyers

When you need expert in Spanish banking case:

File pericial with claim (Art.336): Must include curriculum, declaration of independence, methodology, annexes of Banco de España Circulars.

Request perito judicial if complex (Art.339): Ask judge to appoint professor from CEMFI or Banco de España former inspector - more weight than party expert.

Pre-trial expert meeting: Spanish courts now order careo pericial - both experts meet to list points of agreement/disagreement before hearing.

At hearing: Expert must ratify, explain in plain language for judge, answer: What norm breached? How damage calculated? Causation?

Challenge opponent expert: Ask about fees (bias), lack of specific certification (e.g., no PSD2 certification in phishing case), reliance on bank internal data without independent verification.

Failure to validate: If you present bank employee as expert, court rejects under Art.340 LEC - lack of impartiality. Must hire external.