Tube drivers call in sick almost four times more than other Londoners, taking nearly three weeks off every year compared to the average of three-and-half days for other workers in the capital.
Figures from the Office of National Statistics show that nationally British people each took 4.4 days off in 2014, while Londoners took just 3.5 days.
But statistics revealed by Transport for London in response to a Freedom of Information request from ITV News revealed that drivers report sick on average more than 1,000 times a week, taking 14 days a year per person.
Union officials explained the discrepancy by saying that drivers were banned from working while taking medication because of the nature of their job.
"London Underground have a policy that drivers can't book on if you're on under any prescribed medication at all," Steve Hedley from the RMT union said.
"You can imagine if somebody's on medication it makes them drowsy," he added. "If that person is working in a shop, perhaps you'll get the wrong change, if it's a driver then lives are at risk."
London Underground's chief operating officer did not dispute the figures, but said "in the last 12 months our drivers had had an attendance rate of over 94%.
"We expect our employees to attend work regularly and on time in order to maintain our high standards of service and safety," he said.
If staff fail to meet the standards required it "will result in disciplinary action or termination of service," he added.
The disclosure prompted fresh calls for driverless trains to be introduced from Conservative mayoral candidate and current member of the London Assembly, Andrew Boff.
Calling the sick day levels "unacceptable", he told ITV News that management "should be getting to grips with this absenteeism and addressing the issue more clearly".
"Of course the long term solution actually is driverless trains," he said.
The news comes after plans for the Night Tube were put on hold, as Transport for London could not reach an agreement with unions about changes to pay and working conditions.
The 24-hour service was supposed to begin this week but a series of strikes and the threat of further industrial action on Tuesday and Thursday led to further negotiations and a new start date has not been agreed upon.