The Northern Line was suspended last night after drivers voiced fears about the emergency braking system on the trains after a number of failures over the past week.
No timescale has been given yet by London Underground bosses as to when the line will reopen.
The row escalated when four workers were sent home after they refused to drive trains.
Aslef warned that it will ballot its members on the Tube unless the drivers were paid by 12 noon today.
There have been problems over the past week over the emergency braking system which is supposed to "trip" if a train passes a signal at danger.
LU has been investigating the problem and trains have been double crewed, leading to huge delays in the past week.
A spokesman for LU said: "We are working with our trade unions and staff to reassure them that it is safe to continue to run Northern line trains."
The spokesman said the company was working with the private sector firms which maintain the Tube to resolve the issue.
"We are working on a solution to improve the maintenance of Northern line trains."
Unions said there was a fifth incident of the braking system not working properly on Tuesday night.
The four drivers said they were not confident about taking the trains out and were sent home without pay.
"We will support any of our members who do not take trains out on safety grounds," said a spokesman for Aslef.
The Rail Maritime and Transport Union said it would support any of its members who refused to take trains out and it urged LU to withdraw any trains which had experienced problems with the system.Reuse content