The company’s offer prompted about 40 members of the union, which claimed to have 8,000 KMB employees in its fold, to block buses at the exit of the Sha Tin Central Bus Terminus for 30 minutes on Tuesday morning. Traffic in the area was disrupted.

Lai Siu-chung, leader of the union’s KMB branch, described his employers as “arrogant” and “disrespectful” in the negotiation meeting on Monday. He claimed a management employee had told one of the union members to “shut up” after the member said workers’ pay and benefits since 2014 had not been “ideal”.

“We are strongly dissatisfied with the management. It has dragged on for four months,” he said of the negotiations.

Lai said the union had earlier issued 7,000 questionnaires to its members and most of them opposed the company’s proposal, but management rejected the survey results.

Lai’s organisation is affiliated with the Federation of Trade Unions, a pro-Beijing group with close ties to the government. It is also one of the unions recognised by KMB.

He apologised to passengers on Tuesday for the disruption but threatened to escalate the union’s actions if the company ignored its demand.

“This action was just an alarm. I warn that we will take a more radical step next time, a very radical action. We will not warn the company in advance or tell the public,” he said.

He added the impact of future action could be “very shocking”, without elaborating. Another union, the Full-time KMB Driver Alliance, told the Post that it would join in if necessary.

Bus passenger Cheung Sze-chun, going to Tsuen Wan, said the protesters interrupted his journey for a little more than 30 minutes. Cheung said the industrial action had forced him to get off the bus and wait for a while, adding it had affected those who were in a hurry.

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“I understand they have their demand. But this method is a bit inappropriate.”

The tragedy shook the city and sparked debate over the well-being of 13,000 franchised bus drivers, who have for years complained of being overworked and underpaid.

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But KMB said it had been communicating with unions with a sincere approach. It said its proposal together with the pay restructuring in March and other improved welfare policies would give drivers an average monthly pay rise of about 15 per cent this year to about HK$23,000, if they worked 10 hours a day and their 13th month pay was taken into account.

Starting from March, full-time KMB drivers have been guaranteed a basic monthly salary of HK$15,365 (US$1,963) regardless of performance. Most drivers had already been receiving that amount, but HK$3,556 of the sum was classified as bonus pay, and dependent on whether they met monthly safety and service requirements.

Cheung Tsz-kei, the union's vice-chairman, however, said the company had “exploited” drivers for too long, saying the earlier policies and the pay rise this year were two separate issues.

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The Labour Department said it was closely monitoring the matter, and urged both sides to get back to the negotiating table.

Meanwhile, the Citybus Limited Employees Union also had a pay-rise negotiation with its company on Tuesday. Its union leader Henry Hui Hon-kit said there was a deadlock in talks because the company only proposed a pay increase of 4.2 per cent while they wanted a rise of seven per cent.

He said the process this year has been “very slow”, adding that a deal could be struck before July in the previous years.