Bank customers want more staff in branches amid unhappiness over the reliability of machines, according to new research.
Unite said a survey of more than 1,000 adults found that almost three out of four had suffered because bank machines were out of order.
The findings follow huge problems yesterday when Lloyds Banking Group customers were left unable to use their cards or withdraw cash for hours following an IT glitch.
Debit card transactions were disrupted for three hours and cash machines for four and a half hours before the difficulties were resolved, hitting customers of Lloyds, TSB, Halifax and Bank of Scotland.
The bank apologised to its customers after it was revealed that around half of its 7,000 ATMs were affected.
The Unite survey, carried out earlier this month, showed that almost two thirds wanted to see more staff in bank branches, while one in five were unsure about how to use a machine.
One in 10 said they had been hit by fraud because of hacking or tampering with machines.
Dominic Hook, Unite national officer, said: "The speed that UK banks are moving towards increased automation across their branches is incredibly significant for all bank customers. The poll shows the significant reputational damage the banks will suffer if they ignore the needs of their customers.
"With 72% stating they have had problems with machines in branches and 62% preferring more staff in branches, there is no doubt about the importance of front line bank staff.
"The irresponsible drive to cut staff and save money by installing machines across high street branches will have serious consequences for the banks. The banks need to strike a careful balance between the high quality customer service provided by front line staff and the banking functions machines can perform.
"This evidence that customers want well-staffed branches and would prefer to deal with the highly trained and professional staff that currently service their branches should set off alarm bells for the industry. Their customers don't want to see empty, soulless branches.
"There are serious questions as to what this reduction in staff numbers will do to customer service across the banking industry. It is now time for the industry to rethink its automation plans and give consumers the better basic banking they clearly desire."
TSB chief executive Paul Pester said in a tweet that hundreds of people were queuing to talk to customer services about the issue.
"At one point we had 300 customers queuing to speak to the call centre - sorry if you couldn't get through," he said.
He added last night: "The cause of the problems at TSB is now fixed. It'll take a while to sort the backlog.
"Thanks to all TSB colleagues who worked so hard this afternoon to fix the IT issues. Sorry once again to all our customers affected."
He blamed the problem on a "server failing" in the UK and confirmed all brands under Lloyds Banking Group were affected because they share the same IT system.