Money sent using 'instant' bank transfers yet to arrive two days later
Some "instant" payments which were made by people on Sunday have still not gone through two days later.
Banks and building societies use the Faster Payments system - which means that when people make bank transfers the money is usually available almost immediately, although it can sometimes take up to two hours.
But a hitch on Sunday has caused delays to some payments made between 1pm and 5.30pm on that day.
Around three quarters of a million payments would normally be made during that time period.
It is thought around 1% of payments made during that time on Sunday were still outstanding on Tuesday - meaning thousands of payments could potentially still be affected.
Faster Payments said it is still working to determine the precise number of outstanding payments and they will be processed as soon as possible.
The issue meant a limited number of payments were "rejected" rather than queued, meaning they did not pass through the central payment infrastructure, where money is moved between banks.
These payments now have to be manually checked to ensure they will not create a double payment.
Faster Payments has apologised to customers and said those who may have incurred charges, from being pushed into their overdraft for example, should speak to their bank, which will put matters right.
It said it is still investigating the cause of the issue, and together with its infrastructure supplier, Vocalink, it is prioritising work to process the outstanding payments.
Faster Payments said the system has been working to full capacity since Sunday, so any payments which have been sent since would go through as usual.
The BBC website reported that one man was paying an overdraft charge after transferring £3,000 from his savings account to his day-to-day account, which was yet to arrive.
A Faster Payments spokesman said: "Following the intermittent issues on Sunday with the central Faster Payments infrastructure, we are aware that a limited number of payments that were submitted between 1pm to 5.30pm are still outstanding.
"We would like to sincerely apologise to all affected customers - we are working hard with our technology supplier and all of our participating institutions to ensure the outstanding payments are processed as quickly as possible.
"No-one who has been affected by this issue will be left out of pocket - the industry has a collective agreement to make sure customers are protected financially in situations like this.
"Anyone who has been affected by a payment that hasn't been received should speak to their own bank, building society or other provider."