Royal Bank of Scotland customers have been left unable to file PPI complaints on the final day to lock in a claim.
Millions of people have until 11:59pm on Thursday to claim a refund for mis-sold insurance - with £10billion still yet to be paid out to those affected by the banking scandal.
"This is the final chance for consumers to think about whether they had PPI and submit a complaint directly to any providers right away … There’s still time to act if you do it now," the Financial Conduct Authority has warned.
However a deluge of complaints in the final hours before midnight has caused some banks to crash under pressure.
Royal Bank of Scotland has been experiencing ongoing issues with its online claims form. The page has been down for at least 24 hours.
When Mirror Money contacted the bank, it said the page is fully working.
Customers are instead having to manage their complaints via the telephone or through other services such as Resolver.co.uk - which has received more than a quarter of a million claims in the past 24 hours.
Many of the biggest claims management companies have already stopped accepting PPI claims because, they said, there was a risk they would not be able to register them with the lenders before the deadline.
Santander's PPI enquiry form has also been intermittently "temporarily unavailable" while customers of other banks are reporting website issues and waits of up to 90 minutes on the phone.
If you're having issues accessing your bank's website, consumer expert Martin Lewis says you should take screenshots to prove that you tried to claim before the cut-off point. Time stamps and dates will also support your case.
If you've made a claim over the phone, ask for details such as a reference number and a copy of the transcript of your call.
"As predicted, there is unprecedented last-minute demand for reclaiming PPI. We've seen our own traffic to our tool increase by over twentyfold in the last two days since we started the media campaign. And that huge demand is causing even major bank systems to creak and struggle," Martin Lewis explained.
"The obvious first advice is to take time, be prepared and be patient. However, it is simply not fair or right that people who have tried to submit a claim before the deadline and have been disenfranchised by bust bank tech miss out.
"Take a timed screenshot if possible of any page that isn't working, and if not at least take notes of what you did and when you tried, because almost certainly the regulator will have to ensure that people who miss the deadline because of bad bank tech will still be able to get their claim in, and we will be pushing for it to do so."
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