The taxpayer-owned bank came bottom of league tables published by the Competition & Markets Authority (CMA), based on a survey of personal and business banking customers.
RBS is joint bottom of the personal banking league table alongside Clydesdale, with less than half of customers saying they would recommend the lender.
It also came out bottom for business banking.
RBS has been dogged by several scandals since its Government bailout in 2008, including for the mistreatment of small businesses and most recently a bank branch closure drive.
A spokesman for the bank said: “We are aware we have more work to do in order to improve our service standards and deliver a better experience for our customers.
“That is why we are investing in improving the products and services we offer our personal and business customers, whether that’s through launching initiatives such as the UK’s first paperless mortgage or ESME, our digital lending platform for SMEs, which are helping us to deliver better service for our customers.”
First Direct came top of the personal banking league table with 85% of customers satisfied, while Handelsbanken topped the business ranking with 84%.
Under new rules that came into force on Wednesday, banks must publish information on how likely people would be to recommend them - including online and mobile banking, branch and overdraft services - to friends, relatives or other businesses.
The results must be displayed prominently in branches as well as on websites and apps.
The CMA said it will make it easier for people to find out if another bank has a better offer and will drive up competition.
Adam Land, senior director at the CMA, said: “For the first time, people will now be able to easily compare banks on the quality of the service they provide, and so judge if they’re getting the most for their money or could do better elsewhere.
“This is one of the many measures - including Open Banking and overdraft text alerts - that we put in place to make banks work harder for their customers and help people shop around to find the best deals for them.”
How does your bank rate for service?
Customers with personal current accounts were asked how likely they would be to recommend their provider to friends and family. The results show the proportion of customers of each provider who said they were “extremely likely” or “very likely” to recommend each service.
1 - First Direct 85%
2 - Metro Bank 83%
3 - Nationwide 73%
4 - Coventry - 68%
5 - Santander - 64%
5 - Barclays - 64%
7 - Halifax - 61%
7 - Yorkshire Bank 61%
9 - Lloyds 60%
9 - Natwest 66%
9 - TSB 60%
12 - Bank of Scotland 57%
13 - HSBC 55%
15 - Clydesdale 49%
15 - Royal Bank of Scotland 49%