The problems started early in the morning, with more than 1000 people reporting the issue on complaints tracker website Downdetector.
It comes after Lloyds, Halifax and Bank of Scotland customers were unable to access their accounts for hours last month after servers went down.
The outage appears to be a widespread problem and is being reported across the UK.
Here’s what you need to know about it.
Why is Santander down?
People have been left unable to get into their online banking or mobile banking apps from Santander, but the problem isn’t reserved to one bank.
HSBC, TSB, Natwest and Halifax customers have also reported access issues, as well as those who use First Direct and Marks & Spencer banks, which are both run by HSBC, The Co-operative Bank, Lloyds and Royal Bank of Scotland.
All of the above lenders’ internet and app services were down on Friday morning, with customers reporting issues from as early as 3am.
Over 700 Santander users have complained about a service outage and 331 HSBC customers, 437 TSB users and 100 Natwest customers also flagged problems.
One user commented on Downdetector: "Been trying since about 3am my car insurance is due today so hopefully it still gets taken out, and need petrol to get to hospital today."
The widespread problem is due to an ongoing technical failure which is linked to Sky Broadband and Wi-Fi.
A Sky spokesperson told The Mirror: “Sky Broadband customers may be experiencing issues when trying to access some websites this morning. We're investigating this as a priority and we're sorry for any inconvenience.”
M&S Bank replied to some customers on Twitter saying the issues were connected to a Sky internet outage.
"We are aware of issues for our customers, if you use Sky as your internet provider," the bank replied to one customer.
"We are working closely with Sky to find a solution, so please bear with us."
Many people said they were receiving an error message saying their session had timed out and the server was not responding when they were trying to access their online banking.
What areas are affected?
The outage appears to be widespread, with customers all over the UK reporting issues with their online banking.
A Downdetector live map shows that people living in large metropolitan areas such as Glasgow, London, Manchester and Birmingham are affected, as well as people living in more rural areas.
How can I check if I’m affected?
You are able to check whether you have been impacted by the outage using your bank’s dedicated service status web page.
It will usually show if any services are experiencing issues, including mobile and online banking and card payments.
This can usually be found in the customer support section of your bank’s website.
For example, Santander’s website is currently reporting issues with telephone banking.
Banks regularly issue updates about their services on social media, so it is worth checking Twitter to see if you can get any further information.
Websites like Downdetector also tell you whether other people are experiencing similar issues with an online company.
If you’re a Sky customer that is unable to access digital banking, you could try disconnecting your Wi-Fi and using your mobile data to connect instead.
You should still be able to access your bank’s telephone banking services if you can’t get into your app or website.
Can I claim compensation if my online banking is down?
While banks don’t have a fixed compensation scheme for any service problems, you may be entitled to some money bank depending on how much the issue has affected you.
It is a good idea to gather evidence of your problem, including exactly when you were unable to access the website or app, so you are able to make a formal complaint to your bank.
You could also include any costs you had to pay as a result.
Furthermore, if your credit rating has been affected by a server outage if you were charged a late payment fee after being unable to carry out a transaction or pay a bill, you should also keep a record of this.
Which? has a guide on how to gather evidence if you plan to make a claim for compensation as a result of online banking problems.