CaixaBank received the greenlight from regulators in March to buy state-owned lender Bankia for EUR4.3 billion, creating Spain's biggest domestic bank. Now, it is gearing up for the largest technological integration seen in the country's financial system.

Beginning when branches close on Friday and concluded by Monday, the operation will transmit the information of 7.6 million Bankia customers with a volume of 10.4 petabytes, which is equivalent to 45 times Spotify's stored capacity or 1.8 billion songs in high quality MP3 format.

In total, there are 2.5 billion digitalised documents that, spread out on paper, would occupy the area of more than 20,000 football pitches.

Once the process starts, Bankia’s website and app will only allow customers to check their balance. Once the process is completed, both will automatically redirect to CaixaBank's digital channels.

Bankia customers will then be able to log in with their current credentials before carrying out security checks requiring their mobile phone. They will also be offered an online tutorial on using CaixaBank.