Thousands of London bus drivers, who transport over seven million people every day, started a one-day strike over payment and working conditions Tuesday.

About 27,000 drivers of the double-decker red buses, who work for 18 different companies, walked out at 04:00 local time calling for a collective pay deal among all bus companies, according to Transport for London, the authority responsible for transportation in England’s capital.

Transport for London added that only 44 of 673 bus routes are expected to operate in London on Tuesday and passengers were asked to follow the latest updates on the authority’s website.

Unite union, the largest trade union in the U.K. and Ireland with around 1.42 million members, said there are more than 80 different pay rates covering bus drivers across London. Some companies pay £9.30 ($14) per hour while others pay £12.34 ($18.6) an hour.

“Strike action is the last resort,” Unite union London regional officer Wayne King said. “We have been forced into this position by the operators’ refusal to even meet with us.”

“Passengers sitting side by side on the same route expect to pay the same fare, so why shouldn’t drivers expect to be paid the same rate?” King added. “The bus operators need to stop pleading poverty in defending pay inequality and collectively start negotiating about a fairer deal for London’s bus workers.”

Transport for London said that “services during the day on Wednesday Jan. 14 should operate as normal.”

The Unite union held its first strike on Dec. 29, 2014 and traffic was affected until the early hours of Dec. 30, 2014.

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