Uber is to introduce a cap on the number of hours its drivers are allowed to work.
Under new rules to be introduced next week, a driver must take a six-hour break after 10 hours of working.
The ride-hailing app said the measures were an “industry first” that would enhance safety for both drivers and passengers.
But unions have slammed the change as “tinkering” that fails to address the reasons for long hours worked by many drivers on the app.
“The reason drivers are fatigued is because they are not earning enough,” said James Farrar, the chair of the private hire drivers’ branch of the Independent Workers’ Union of Great Britain.
“If Uber forces drivers to work less without paying more it is just going to depress hourly earnings even further below the minimum wage and push drivers to the brink.”
Research by the IWGB, which represents workers in several areas of what has come to be known as the “gig” economy, found Uber drivers had to work just over 34 hours a week to break even.
The same findings suggested a net hourly income of £5.17 per hour.
The measures mean the app will enforce a six-hour break demand after a driver racks up 10 hours driving on a trip with a passenger, or on the way to pick one up.
If a driver is waiting for a request, sitting in an airport queue or driving without a passenger, however, this will not be included in the 10-hour limit.
Uber says that nearly a third of its 50,000 UK drivers are logged into the app for more than 40 hours a week, with just under 8% online for more than 60.
“Licensed drivers who use our app really value the freedom and flexibility to choose if, when and where they work,” said Andrew Byrne, Uber’s head of policy.
“We continue to listen to feedback and plan to make other changes and improvements over the coming months.”
The ride-sharing app has in recent months been plagued by concerns over safety and driver rights.
In October, Transport for London denied it a licence to operate in London, saying it had demonstrated a “lack of corporate responsibility”.
The following month Mr Farrar and another driver won a landmark case against the ride hailing app, with a court ruling they were workers entitled to the minimum wage, sick pay and benefits.