Some customer have claimed to being able to see other people's bank details while one user said his account was showing the wrong sum of money.
TSB customers have taken to social media to vent their frustrations over the issues after TSB had used Twitter to warn customers of upgrades over the weekend.
Their tweet on Friday said: "We’re planning an upgrade to our systems between 4pm Friday 20th of April until 6pm Sunday 22nd of April.
"We’re sorry but during this time some services, like online banking, making payments or transferring money won’t be possible."
Hundreds of tweets have been sent to TSB's official Twitter account highlighting issues.
The firm have issued a statement.
A spokesperson said: "Unfortunately, there are some intermittent problems affecting these services.
"Both the app and internet banking should be up and running again soon."