London bus drivers are so exhausted they are at risk of being a danger to passengers and other road users, a union has said.
Unite plans to ballot its 20,000 members and said if industrial action was backed, drivers would strike "later this year".
A spokesman said drivers were suffering "chronic fatigue" and had "had enough".
Transport for London (TfL) said it was "such a complex and multifaceted issue there [was] no quick single fix".
A survey of bus drivers carried out last year found that 17% of those who responded had fallen asleep while driving the bus at least once in the past 12 months.
The survey was part of a report carried out by Loughborough University into bus driver fatigue in the capital.
Of the 1,353 drivers who completed it, 21% said they had to "fight sleepiness" at least two to three times per week, while 36% said they had experienced a "close call" in the past 12 months because of fatigue.
The union has now called for "decisive action", including proper breaks and making sure drivers finish their shifts on time.
Unite regional officer John Murphy said: "London bus drivers have had enough. They are permanently fatigued and at risk of being a danger to other road users, bus passengers and themselves.
"Unite will shortly be conducting a consultative ballot and provided our members endorse industrial action, strikes will follow later this year unless action is taken to ensure that the problems causing chronic fatigue for our members are resolved."
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Claire Mann, director of bus operations at TfL, said: "Safety is our top concern and we are working hard to eliminate death and serious injury from our roads.
"Last year we published our world-first driver fatigue report which includes a variety of tough measures to tackle the issue.
"As this is such a complex and multifaceted issue there is no quick single fix, and the whole bus industry has a part to play.
"We urge Unite to continue to work with us as it is only through collaborative action across the industry that we can address this vital issue and make our buses lead the way in reducing risk on the roads."