The union said its members, employed by Arriva North London, voted against an offer to resolve the row, adding that the offer did not go far enough to solve the problems of hot weather working.
Unite general secretary Sharon Graham said: “London bus drivers do a vital job, but are putting themselves at risk due to these appalling conditions.
“Arriva can fully afford to make meaningful changes our members can accept.
“Until that happens, strike action will continue and our members at Arriva North London will continue to have the full backing of Unite.”
Lorna Murphy, director of buses at TfL, said: “We know that any disruption to London’s bus network can have a significant impact on our customers, and we’re working closely with Arriva to help customers travel during these periods of strike action.
“We encourage anyone travelling in the affected areas to plan ahead, allow extra time for their journeys, and check before they travel using the TfL Go app or our website, external. We’re sorry for any disruption this may cause to our customers.”
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