The banking arm of supermarket chain Tesco could be facing a fine of more than £30m from the Financial Conduct Authority (FCA), over its role in a cyber hack on its systems in which numerous customer accounts were compromised.

Tesco Bank is thought to be contesting the fine in active negotiations with the regulator, which could be reduced to a "substantially lower" number within the next few weeks, sources told Sky News this evening.

However if it goes ahead, the fine will be the largest ever issued by the FCA in cyber security.

Tesco's retail banking systems suffered an attack from hackers in November 2016, after which the retailer was forced to notify 9,000 of its customers and provide a total refund of £2.5m.

Read more: Government agency launches criminal investigation into Tesco Bank attack

Tesco Bank and the FCA declined to comment.

The bank has over 6m customers in the UK, and posted a profit of more than £200m in its last financial year.