Computer glitches mean many customers of three major banks cannot access their accounts online today.
Many Barclays, Halifax and Bank of Scotland (Hbos) account holders won't be able to check balances and transfer money via their computer.
Barclays and Hbos say the problems are sporadic so while some won't be able to access the website at all, others will have no problems (see the Bank Accounts guide).
Banking sources suggest the huge volume of people checking their accounts on the first working day after the Christmas and new year break is the likely cause of the problem.
The glitch has hit part of the Hbos arm of the Lloyds Banking Group which means Intelligent Finance customers could also experience delays, though Lloyds TSB customers are unaffected.
Anyone who receives a bank charge as a direct result of not being able to log-on and transfer money will have any refund request dealt with on a "case-by-case" basis.
It is still unclear when the problem will be rectified. Hbos adds its telephone and branch services are working normally.
An Hbos spokesman says: "Some customers may be experiencing problems logging in to Hbos online. We are aware of the issue and are working to resolve the problem. Our branch and telephony services are unaffected."
MoneySavingExpert tried to log into the Halifax website at 2.30pm today, but the homepage did not load at all, let alone the account sign-in screen.
We had no problems getting to the Bank of Scotland, Birmingham Midshires or Intelligent Finance sign-in screens but we didn't attempt to sign-in.
Barclays says its online banking system was "slow" this morning. Many customers have complained on our forum of being unable to access their account at all.
We had no problems getting onto the Barclays homepage at 4pm today but we couldn't load the account sign-in page.
Instead, we got the message: "We're currently experiencing technical problems in Online Banking that mean some of you may not be able to log in to the service.
"We apologise for any inconvenience this may cause and suggest you try logging in again later."
Like Hbos, Barclays says its telephone and branch services are unaffected.
Barclays also states any bank charges refund requests will be dealt with on a "case-by-case" basis.
A Barclays spokeswoman says: "We've restarted our servers and it appears response times are returning to normal."
Further reading/Key links
Top accounts: Best Bank Accounts
Correct bank errors: Financial Fight-Back