LONDON (Reuters) - After a ransomware attack on financial data firm ION affected Intesa Sanpaolo's derivatives operations, the bank says most of the affected activity has been restored.

The 'cleared derivatives' section of ION Markets UK had come under attack on January 31. It had spread to all derivatives markets, blocking trading and clearing of exchange-traded derivatives at some of the world's largest banks and financial institutions, including Intesa Sanpaolo.

"We promptly took measures to avoid a complete suspension of activities related to the exchange-traded derivatives segment," a bank spokesperson told Reuters. "Most of the activity has now been restored."

An ION spokesman said some customers' services were reconnected on Tuesday and work continued today.

(English version Claudia Cristoferi, editing Andrea Mandalà)