The bank's @NatWest_Help account tweeted: "We're aware of some issues on our mobile app and online service and are working hard to fix them".
After the issues were resolved, the bank put out a statement which read: "We're very sorry about the technical problems our customers have experienced today".
Customers have been reporting issues since this morning, and it seems the bank is still struggling to fix the problem.
The Co-operative Bank and Cashplus also suffered outages this week. Hannah Maundrell, editor in chief of money.co.uk, said: "Banks really need to pull their socks up because this keeps happening again and again".
"We're still working through what the issue is".
It comes after City A.M. yesterday reported that Barclays faces a Treasury Select Committee investigation into a failure that saw customers unable to use its online banking services for six hours earlier this week.
CUSTOMERS OF NatWest, RBS and Ulster Bank have been locked out of their online and mobile accounts.
According to NatWest, you can still access your account on the telephone and on ATMs.
'We would like to apologise for the inconvenience.
NatWest customers took to social media to complain about the issue, saying they would be unable to pay their bills or access their money. This morning a spokesman for the taxpayer-backed lender said that it had resolved the problems and that all its online services were back up and running as normal. Not really acceptable for our 24 economy!
Commenting on the Barclays outage, Lev Lesokhin, SVP strategy & analytics at software vendor Cast, said United Kingdom banks seem to be unusually vulnerable to such glitches, which are often due to software failures.