Furious Think Money customers have been locked out of their accounts since the end of last week after issues with the banking provider's IT system upgrade sent online services into meltdown. About 100,000 customers have been affected and if you've lost out you may be able to claim compensation.

Think Money, which promotes itself as "the smart alternative to a bank account", provides a personal account that ring-fences money to cover regular bills and other financial commitments so that customers know exactly how much they've got left to spend each month. Customers pay a fee of £17.50/mth for the service.

All of Think Money's 100,000 customers were left unable to access their online accounts or use the bank's app or text services last Thursday (17 November) after a "major upgrade" to the firm's IT system inadvertently caused chaos.

Customers continued to be locked out of their accounts over the weekend and into today, with some reporting they've been unable to access recent payments made into their accounts.

Around 8,000 of the Manchester-based firm's customers were also affected by an issue earlier today which meant income received on their accounts wasn't immediately credited to their cards – meaning some could have experienced difficulty making a withdrawal. Think Money says this problem has now been resolved.

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Think Money customers plunged into 'complete chaos'

A number of angry Think Money customers have contacted MoneySavingExpert.com and taken to social media to vent their frustration:

Another customer, Kerri, emailed: "I've used [Think Money] for a good few years. On Thursday at 11am they began an 'upgrade' to their services online and notified customers that their website would be out of action for 24 hours.

"This then turned into another 24 hours and has continued throughout the weekend, leaving customers in distress with no money due to their whole financial system being in complete chaos."

When will the problems be resolved?

Think Money says accounts were accidentally taken out of action on Thursday following the company's IT upgrade, with the issue continuing over the weekend and into today. All of Think Money's 100,000 customers were unable to view their finances.

About 8,000 were hit this morning by a separate issue which meant income which came into their accounts and wasn't ring-fenced to pay future bills didn't automatically move across to their cards, meaning some were unable to access funds as they were supposed to.

Think Money says this issue has now been resolved and those affected should have received a text informing them of their balance.

A spokesperson told us: "Think Money customers have been unable to log in to our online service or use our app or text services. We have now restored those services, but we are restricting the number of customers that can use them at any one time, due to very high demand. We are working round the clock to restore our digital services fully."

Think Money says payments were received and made as normal during the period customers were unable to log on to their accounts. It says its online account service should be "back up to full speed" by the end of the day (21 November).

Can I claim compensation?

If you've been left out of pocket or otherwise inconvenienced by the issues at Think Money you may be able to claim compensation, although Think Money has told us it plans to pay it on a "case-by-case basis".

If you'd like to enquire about claiming compensation you can log your complaint by calling 0161 779 5000 - if the Think Money call centre staff are unable to deal with your complaint you can request to instead speak to the complaints resolution department.

If you feel as though your complaint hasn't been handled properly or you want to raise it in writing, you can use the free online Resolver complaints tool*, which will help you draft a letter, send it, monitor replies and then escalate it to the Financial Ombudsman Service if it's not sorted.

What exactly is Think Money?

Think Money is an independent company, though it pays Royal Bank of Scotland to hold its customers' cash in dedicated accounts. It's essentially a 'jam jar' banking provider, which helps manage your money in return for a monthly fee.

Customers' wages, benefits and tax credits are paid into a Think Money account – the company then sets aside the money customers need to cover bills and other regular outgoings so they know exactly how much they've got to spend each month. Customers are able to go 'in branch' at the Post Office.

It charges a monthly admin fee of £17.50, but the company claims this is offset by the fact that customers can't get overdrawn (and incur subsequent charges), nor are they charged transaction fees for payments such as direct debits and standing orders.