Many customers of Northern Rock were unable to get reassurance from the company's website during last week's crisis when it was forced to get an emergency loan from the Bank of England to prevent it breaching solvency rules.
The mortgage lender's loan follows the US sub-prime mortgage crisis, which has gripped the financial world.
Following media reports branches were swamped by customers and the website was inundated with requests.
Following the website problems, Northern Rock said in a message to customers: "Thank you so much for your patience, particularly when using our website, which has been running very slowly due to the number of people working online."
This problem has highlighted the importance of a corporate website having good load-balancing technologies during a crisis of this type, where accessing a website is sometimes the only means of customers getting information fast to prevent heightened panic.
Hackers often use techniques such as distributed denial of service attacks to crash websites through high volumes of requests, to damage corporates.
The FSA said the problems of the website were caused by the high number of customer requests. "Clearly, there have been some operational problems with queues at some branches and difficulties with the bank's website caused by the unusually high volumes of customers trying to access their accounts as a result of the publicity surrounding Northern Rock," said an FSA statement.
Callum McCarthy, chairman of the FSA, said, "To be absolutely clear, if we believed that Northern Rock was not solvent, we would not have allowed it to remain open for business. It is open for business and it can continue to receive deposits and allow customers to make withdrawals."