Slaughters corporate partner Roland Turnill (pictured) has been instructed by the Spanish bank, along with associates Stuart Whitty and Victoria Ballantyne.
Turnill has forged close links with Santander since being made up in 2004, representing it as part of the Royal Bank of Scotland (RBS)-led consortium that bought ABN Amro for £49.2bn.
Uria Menendez fielded a team of three partners - Salvador Sánchez-Terán, Javier Redonet and Antonio Herrera - for the rights issue, which will see 1.6 billion new shares offered to shareholders at a significant discount.
Santander had previously been seen as unlikely to need extra capital, emerging as one of the strongest European players in the banking sector.
Davis Polk's New York-based corporate partner Nick Kronfeld represented the bank in the US.
The issue is being led by Merrill Lynch and is fully underwritten. Linklaters Madrid banking partner Inigo Berricano is advising Merrill Lynch in Spain, the UK and the US.
Shares in Santander fell five per cent in response to the news.