An employee at recruitment agency Hays accidentally emailed the rates that RBS pays for contract staff to 800 of the bank’s employees.
The email, sent to remind RBS managers to fill in time sheets for staff contracted from Hays, also contained rates for 3,000 contractors, some of whom receive £2,000 per day.
“We are extremely disappointed that confidential personnel data has been shared by one of our suppliers," RBS said in a statement. "This is unacceptable and we are taking action to address this issue."
"No customer information has been compromised," it added.
Hays says it has apologised to RBS for the error, and that it managed to recall half of the emails before they were read.
As a result of the leak, RBS was criticised by trade union Unite for "throwing money at thousands of contractors" at the same time as making job cuts.
"We have expressed to the bank our unhappiness that thousands of permanent staff are being sacked when contractor staff continue to work in other areas of the bank. Instead the bank should be investing in its staff through retraining and upskilling," said Unite national officer David Fleming.