The National Crime Agency (NCA) has launched a criminal investigation into the recent fraud attack on Tesco Bank.
The National Cyber Security Centre (NCSC) said it was working with the NCA and Tesco Bank, as well as other groups, and said it was "providing direct assistance to the company at their request, including on-site assistance".
The challenger bank was forced to halt online transactions yesterday as it moved to deal with the incident.
"In the case of cyber related incidents, it can, on certain occasions, take a significant period of time to understand the incident given the technical complexities involved," the NCSC said.
"So the story will emerge over time. During this period it is vital that nothing is said publicly that could interfere with the criminal investigation.
"Given the investigation thus far and the evidence at hand, the National Cyber Security Centre is unaware of any wider threat to the UK banking sector connected with this incident."
Treasury select committee chair Andrew Tyrie blasted banks over their cyber security arrangements in the wake of the fraud attack.
“This is just the latest in a long list of failures and breaches of banking IT systems, exposing many thousands of customers to uncertainty and disruption," he said. "We can't carry on like this.”