The bank, which has been phasing in an upgrade to its website since January, said it had been experiencing above-average traffic during the final days before the new tax year.
The problems started at around 7:30am and Barclays was “working hard” to rectify them as soon as possible, a spokeswoman said.
She said: “We apologise to customers who may be experiencing slow responses from Barclays online banking today. We are seeing higher than normal volumes of traffic but the service remains available and we are working on the response times.”
Barclays could not give a figure for how many customers have been affected but the number is thought to have changed over the day.